This post nails it. I also thought Artorias was the most fun boss in the game, but man... I remember having to take a long break from the game after having banged my head against the wall for hours. I'm also adamant about never using shields in these games, so it's completely my fault, I'm doing that to myself.

The Capra Demon was a nightmare for me. I had to cheese it by standing on that tiny semi invisible ledge... It was not fullfilling.
I didn't really find any of the bosses easy... Maybe the gargoyles? It was the most fun, if nothing else.

I legit just gave up on Artorias and never thus actually did the DLC, so him? After that was Ornstein and Smough for sure, both times.

Easiest relative to how hard you'd think they should be? Four Kings for sure, I just ran up to each one and killed them in 3 hits each. Both times I killed the easy stuff in one go, butterfly thing, sif etc.

I missed the lock on message very early on in the game, and didn't work out how to lock on until like my 30th Gwyn battle, on my second playthrough (not NG+) I'm batttering the bastards now I have lock on. I managed to parry Gwyn like 1/5 times.

Nito was a bugger he kept doing that thing he does (I struggle with the pursuer for similar reasons)

I one-shotted Four Kings, Sif and Centipede. I still struggle with Hydra, but on my original playthrough before I quit(super cursed) I managed to kill it quite easy, but somehow I can't do that anymore.

I thought Smough and Ornstein were by far the hardest. Manus, Kalameet, Artorias and even Gwyn were all challenging in their own right, but by that point I've levelled up pretty high and with my trusted Black Knight Greataxe and I think either Tower Shield of Black Knight Shield I was pretty much unstoppable.

Let's be realistic, Pinwheel is barely a boss. Dark Souls 2 has a similar glaringly easy boss that is little more than a strong mob with a lifebar too. So I'm not going to count him.

It's gotta be Ceaseless Discharge. It's hands down easier than anything else.

Artorias was the easiest boss for me (I don't count Pinwheel as a boss) and The Capra Demon was the hardest boss for me the first time around.

Smough and Ornstein these two for sure are the worst, they both take tons of damage and they just have so many moves that can end your run.

I beat the 4 kings first try which was one of the best moments I have had in the souls series, I was so tense having no clue what to expect from him everything just fell into place and I got em.

Hardest (First time) - Seath & Bed of Chaos.

Hardest (Subsequent times) - Bed of Chaos

Most Fun - Manus

Surprisingly Easy (compared to hype) - Capra Demon & Gwyn.

Easiest is Pinwheel, I killed him in two shots - Gravelord Nito was also pretty easy, I beat him first time I remember.

Most trouble was Smough and Ornstein first time through, had a lot of trouble with my dex character fighting Artorias as well.

Kalameet was by far the hardest boss for me to beat. Pinwheel or the Butterfly were the easiest for me to beat, and my favorite boss was by far Artorias since it was such a fun, technical and demanding fight.

Just fought Artorias and he kicked my ass for some time. Such a satisfying kill.

Hardest: The Four Kings

Easiest: Iron Golem

Asylum Demon in the first encounter (like literal first time you walk through the doors) is actually the easiest boss. Kalameet is the hardest.

O&S solo (and especially killing Smough first) are hands down the hardest fight across all 4 games. They were designed for coop and it shows, I've never had much fun fighting them on my own. Not Havel and his buddies from 2's sunken king dlc are probably second but again they were intended for coop.

Pinwheel is odd he feels completely misplaced as by the time the average player gets through the catacombs they just steamroll him. Once I tried running to him straight from the asylum and since I couldn't kill him before he started teleporting and cloning he was pretty rough, at least harder than taurus or the butterfly.

O&S were rage inducing solo. I had a tough time with Iron Golem, too, partly because of the limited space that sent me falling to my death more than once.

Seath I found surprisingly easy, though I was probably over-leveled by the time I got to him.

Ornstein and Smough is the fight I spent the most time on, for sure. Artorias is a close second, and Manus a distant third. Maybe I just lucked out, but I killed a lot of the other bosses mentioned here in one my first try. Four Kings, Kalameet, Gwyn, etc.

Easiest: Pinwheel, of course.

Easiest will always be Pinwheel.

The hardest was Manus because I created a special character for the DLC that was at a low level with a small amount of vitality and Manus was able to one shot me. It was the best fight in the game because I had to be ON POINT for the entire fight.

Artorias was killed on my 3rd attempt with this same character.

On my first playthrough the fight I had the most trouble with was probably the Bell Gargoyles. It's pretty funny, because now I can just run in and kill them both in 20 seconds with zero issues, but I must have died to those fuckers like 30+ times the first time around. Nowadays I'd probably say Manus or Four Kings in their later NG incarnations are the most annoying.

Easiest is Pinwheel or Moonlight Butterfly just because I'm usually super overleveled by that point.

I had a really hard time in Sif for some reason...I beat the Carpa Demon after 2 goes.

Had a really hard time with Capra and Ceaseless Discharge first time through. Also didn't enjoy Sif because the inability to 360 roll in this game really screwed me in that fight for some reason. O&S aren't easy but I actually think the boss run up to them is harder than the actual fight. Gwynn is cake once you realize that you can parry him.

Pinwheel isn't just the easiest boss in this game. He's probably the easiest in the entire series.

Playing on a new character-

Easiest so far: Gaping Dragon and Ceaseless Discharge

Most difficult so far: Blighttown >.< Even more so due to making the repeated attempts to grab Power Within. I don't want to just leave the bloodstain containing 19,000+ souls. Sif still gives me a bit of grief. I don't know why I have such a difficult with him.

I just go through the backdoor to Blighttown now. I'm slowly trying to make my way through the game to try and platinum it.

Pinwheel, Asylum Demon and Moonlight Butterfly were probably the easiest bosses. Hardest bosses were Smough & Ornstein.

O&S was the hardest for me, the big arsehole on the roof of Sen's Fortress was the easiest though. Pinwheel doesn't even count. The butterfly was super easy for me too, another 1-shot.

Now that I have gotten further into the game, the easiest is definitely Pinwheel with Nito being a close second. The most difficult, so far, was Artorias.

Ornstein and Smough was far and away the easiest for me like it was harder to kill the first skeleton the first time I played the game. I don't if I was adept at juking or whatever but I got them to charge double side step and basically murdered one then the other not sure I was even touched.

Astorias gave me problems then Gwyn for me on the hard scale.

Annnnnnnnnnnnnnd fuck The Four Kings! I still cannot solo those fuckers. The Havel armor poise tank method no longer works on them.

Having just beaten the game for the very first time last night, I'll do a ranking of them. I don't have the DLC, but after reading all the raving about it, I might just get it... if I can stop myself from buying a PS4 in a couple of days now that I'm done with all the games I'm interested in for the last gen consoles.

Anyway, rankings! They're ranked from the hardest to the easiest based on my personal experience as a Dex-based melee fighter:

EDIT: Got to play the DLC not long after this post. Slotting in those bosses now based on my first go at them. Mind you that I only got to fight them at NG+ with all my equipment and levels so my first experience against them might not be the same for most:

1. Gwyn

I just found out reading through this very topic that you can actually parry him. I probably would have had a much easier time if I knew this since I had a +4 Lightning Rapier! Took me about 8-10 tries to kill him solo with my +4 Quelaag's Fury Sword and +14 Heater Shield. The breakthrough for me was going back to my roots of just dodging his attacks instead of trying to block every single one. I was doing about 150 damage with each swing since I also had 5 Humanity to buff my sword. Probably the most tense fight I've had the entire game! I think I like this fight better than the last boss of Demon's Souls well, not the actual last boss but his soul form, mainly because it was a more challenging one.

2. Artorias

I'm basically seconding just about what everyone has said about how this is the most fun boss fight in the whole game. His moveset, his attack pattern, his damage output, and the area all just work perfectly together to make for a very challenging but very much doable fight with no dumb gimmicks or environmental hazards. I died about 6-7 times against him, because he would take me out in one or two hits with my particular character build. I did, however, make the conscious decision to not just tank him using his own greatshield against him (Greatshield of Artorias has the highest stability and makes all the other boss fights much easier). It was just a straight-up dodge-fest between him and I. Loved learning his AI behavior and getting in the shots that I could while rolling just out of the way of his attacks.

3. Bed of Chaos

Technically, it's more of a puzzle than an actual fight. But goddamn did it take me a while to not get hit and fall into the bottomless pit. Also took around 8-10 tries.

4. The Four Kings

I wore very light armor, used an unupgraded Heater Shield, and my Vit wasn't very high until the end of the game. Basically, it didn't take much for any of these guys to kill me immediately. Thankfully, I also dished a good enough amount of damage myself. Again, dodging the attacks proved to be the answer for me. It took me about 4-5 tries to beat them, and I also had to whack a fifth king a couple of times to finish the fight.

5. Gravelord Nito

The first couple of attempts led me to believe I was going to be stuck against him for a long time, since the only divine weapon I had then was a base Astoria's Straight Sword that I didn't even have enough Faith to wield properly to begin with. Just kept getting overwhelmed by the skeletons. I was only 2 points away from reaching the minimum Faith requirement, so I used some stored souls to hit 14 Faith. On that very next attempt, I just killed a couple of nearby skeletons and let Nito come to me. Switched to my trusty Quelaag's Fury Sword and just destroyed him in a minute. I guess the key was to just not aggro the giant skeletons in the back!

6. Belfry Gargoyles

My dinky +1 Rapier and frail body couldn't handle this fight the first 4 times. Rolling off the roof didn't help. However, using my +4 Short Bow and a ton of arrows did the trick for the 5th time.

7. Quelaag

She decimated me almost instantly the first two times. I had quite enough of Blighttown though by that time (died way too many times just falling off stuff) so I ended up summoning a dude and Maneater Mildred. They got her attention for most of the fight, while I just plugged away at her with my bow. Got the final blows in to finish the fight.

8. Sanctuary Guardian

Although I didn't die when I first fought it, it still gave me a good challenge because of how fast and unpredictable it was. A combination of lightning and poison damage also kept me on my toes a lot. Now if we're talking about the TWO Guardians, they might just be the second hardest fight in the game. Had to cheese my way to victory against those, luring them near the entrance just to potshot one.

9. Seath the Scaleless

Was pretty reckless on the first two tries, trying to just power through the ice attacks since I was doing a good deal of damage two-handing the Fury Sword. When I actually tried dodging the attacks, it was pretty easy.

10. Demon Firesage

Took about 4-5 tries. Timing his flame attacks and reading the range of his club was all it took, really, since I couldn't take too many hits. Nothing fancy.

11. Capra Demon

Killed him on the 4th try with my +4 Short Bow. Getting rid of the two dogs immediately makes it a much easier fight. It's just a matter of luring him up and down the stairs.

12. Sif

Beat the wolf in 1 go, but I was pretty sloppy since I was doing so much damage. Almost killed me numerous times, too.

13. Kalameet

Killed me the first time when I got its life down to half. It was enough time for me to understand its attack pattern. A couple of close shaves, but the Greatshield of Artorias pretty much nullified all of its melee attacks. It was just a matter of moving away to not get hit by its dark magic breath.

14. Centipede Demon

Also beat this monstrosity the first time, but with the help of Solaire to distract it. I took plenty of risks fighting it so close to the lava, and I managed to kill it just before it could kill me.

15. Priscilla

Got killed the first time since I didn't know how to deal with her becoming invisible. The second time around, I saw her footprints. It was a massacre shortly afterwards.

16. Stray Demon

Exact same fight as the Demon Firesage but I got the timing down much faster so I beat him on the first try.

17. Ceaseless Discharge

Killed this thing conventionally so it almost killed me a couple of times. Nothing fancy here either. Just dodged his slams and thrusts.

18.Manus

Had help with two pretty powerful guys and Sif so this fight didn't take long either. Haha. I'll try to solo him with my new character to see how frustrating he can really be.

19. Ornstein & Smough

Although I heard about the difficulty of this battle, I didn't know it was the one coming up when I was going through Anor Londo. So for shits and giggles, I summoned Solaire and a player. The two of them went after Smough while I fought Ornstein. Pummeled Ornstein pretty easily, actually. 2nd form Smough didn't prove to be much of a challenge against the three of us. Maybe if I do play through the game again, I'll fight these two solo just to see how frustrating it can really be. EDIT: I fought them on NG+ solo, and yeah, they're definitely more of a challenge, but it only took me three tries, figuring out the strategy on the second and executing it properly on the third. Basically kited them and killed Ornstein first. Smough was easy enough on his own. I'm going through the game for a third time but with a totally new character. Will try these two again solo without endgame gear.

20. Iron Golem

Basically an easier Tower Knight from Demon's Souls. Beat him to within an inch of his life, then toppled him over the ledge on the first go.

21. Gwyndolin

Got killed trying to rush in with its magic fireball attack the first time. Careful dodging and moving the second time led to an easy win.

22. Asylum Demon

Plunge attacked then dodged most of its attacks. Simple enough.

23. Moonlight Butterfly

Got killed once since I was relying on my Short Bow for that period of the game and didn't have enough arrows to kill it. After stocking up, it didn't take long for it to go down.

24. Taurus Demon

Smashed me once with its jumping attack then the idiot promptly jumped backwards only to fall to its death. It was a pretty fast and hilarious sequence of events.

25. Pinwheel

Yeah.

All in all, I didn't experience a lot of hair-pulling battles. I probably would have had a tougher time had I not summoned against Quelaag and O&S.

Kalameet is the most difficult for me. I just cannot, for the life of me, get the fucker into a predictable attack pattern. I also have a lot of trouble with Sanctuary Guardian. Yes, easy when it does its predictable attack patterns. Like Kalameet, it does random shit without hardly any breaks between. It Super Mario jump stomped me a few times, instantly killing me each time, and it didn't even matter that I was using Greatshield of Artorias.

If you ran with high poise, good armor, a shield and a claymore/spear, nothing was difficult. If you had Havel's, it just breaks the game. Four kings is just mash, estus, almost zero blocking or rolling. Gwyn is a joke since there's that pillar you just kite him around. O&S are only hard if you freak out cause of the 2 on1 other wise its just a long fight (Rewards consistency, instead of luck due to rng). Only fight in the game that punishes you for running Heavy is Artorias, so you take off your chest and helm then laugh to the souls bank.

I did this all on my first blind playthrough except for knowing beforehand that O&S was a 2 on 1 from a friend who recommended the game.

Finished the game today! The most difficult was Manus for me. I couldn't do it without Sif to help me. Easiest was Seath. I had the Cursebite ring on, just chased after his tentacle-feet and whaled on them. (Yes, even easier than Pinwheel - he at least caused me some damage) The most bullshit was Capra. The most fun was Artorias, got him in two. The most boring was 4 kings. Just run, run, run, whack, whack, whack.

Oh, and I got Gwyn on my first attempt. Five ripostes with +15 Uchigatana, and down he went.

I would say Ornstein and Smough with the caveat of playing the game solo. After that, Artorias. Kalameet and Bed of Chaos are really hard in a bullshit way in my opinion.

Easiest? Pinwheel for sure. Gwyn if you can parry.

Centipede was pretty easy. If you head to the right after the fog gate, there's a large area of solid ground with a dead body on it. Sprint there. You'll have time to heal your lava burns before he's on top of you. You'll have a lot more room to maneuver there. Makes it a lot easier.

Finally beat it, mainly with light armor and Uchi so here are my lists with a dirty dex build.

Easiest, not counting Pinwheel because... yeah. Pinwheel:

1. Taurus Demon - Climb the ladder, plunge attack, rinse, repeat. Or just give him a Gold Pine Resin colonoscopy if you're feeling lazy.
2. Ceaseless Discharge - Just hide behind that rock and poke him with a stick.
3. Four Kings - Dash to the first one. Cut him up. Dash to the second one. Cut him up. And so on and so on. I honestly don't remember a fifth one since I was too busy being Raiden.
4. Gwyn - Parry. I took some hits here and there, but still. I had plenty of time learning how to parry in Anor Londo.
5. Bell Gargoyle - Cutting off the first one's tail becomes the greater challenge after a while.

My honorable mention goes to Artorias because of how disappointed I felt after beating him on only my second attempt. Maybe because by the time I got to him, I was soul level 106 with 40 dex, 40 endurance, 36 vitality and a +15 Uchi and thus had an easier time dealing damage, blocking, rolling and not letting him power up more than once. Or maybe it's because I knew too much after watching Vinny and Woolie (of SBFP) fight him and was taking notes, or maybe I somehow just got lucky. Still, since he's been touted as the hardest boss of the game for so many years at this point, I expected a struggle, but I got Vergil instead, which isn't inherently a bad thing, but my expectations for failure were pretty high. So, I'll see how he is in new game plus because I want a rematch to try to get the struggle that I'm missing out on.

And now the hardest bosses for me, not counting Bed of Chaos and that gimmicky-ass boss puzzle bullshit that I don't appreciate in the slightest:

1. Ornstein & Smough - Even with Solaire's help, trying to kill Super Ornstein resulted in me counter healing more often than I'd get a good hit on him, and Solaire can only draw so much aggro since he doesn't last very long either. So, despite getting through this fight, even with Super Smough on a different character, I still don't have a good read on their actions.
2. Kalameet - I probably died the most to Kalameet. He doesn't fuck around, and his reward is just another "fuck you" on top of it. But this is what I expected from Artorias; to beat my head against the wall several times until the wall finally caved in.
3. Manus - Pretty much another fight that I got though by counter healing and having Sif draw aggro, but I did get a better sense of what to do compared to Ornstein and Smough and got very close to beating him on my own.
4. Stray Demon -hits me way too hard before I can figure out it's pattern, and I can't dodge that AoE for the life of me; same with Demon Firesage since it's pretty much the exact same fight, but I think Stray hits harder.
5. Nito - It took me a little while to realize that the big skeletons don't activate if you don't go near them. So, because I tried to slash everything that both moved and didn't move on the first few attempts, he edges out over Quelaag's lava and Capra's dogs.

And my honorable mention here goes to Quelaag because my stats and equipment aren't up to snuff at that point so I need Mildred's help to draw aggro while I tap dat spider booty and gams with Uchi.

Easier : Pinwheel.

Hardest : _ Ceaseless Discharge (haven't won against him without cheating so far),

_ Black Dragon Kalameet (have only tried and beaten him once with my tank knight with a shield and I was accompanied for the first half),

_ Bed of Chaos (had to watch a video)

Those who still pose me a challenge : Demon Firesage and The Four Kings.

I think I get the trick with Artorias : never let him do his dark explosion. But doing all the way to him each time you lose is boring as hell.

I never played the DLC, but I hear Artorias is real tough.

For me, the easiest boss is by far Pinwheel. I don't know what his attacks are because I killed him before he could do anything. The hardest boss for me was Ornstein and Smough. That shit took forever and a half of circling around the room trying to get one poke in on either character.

I'll be honest the Minotaur you need to fight to get the key for the basement area almost made me rage lol. Those sogs use to attack as soon as I walked through the mist. Had no time ti react. Pretty much had to hope and pray they spawned further back from the door.

The final boss was a lot easier than I was expecting him to be.