I'm pretty sure the plurality's going to be San Andreas, but I'm really interested in how it breaks down from there. 
 
EDIT: As for my personal opinion, while I have not played the original top down titles, nor any of the handheld titles other than Chinatown Wars, I have to hand it to San Andreas. What I find particularly interesting is that I didn't play San Andreas until after I had invested a significant amount of time in IV, and I only actually beat it a few days ago. San Andreas still holds up immensely well. While it does suffer from a lot of AI jank and does not hold up visually, the freedom of experience on display and the sheer variety of locations and and environments to explore is still really impressive. I can only imagine how mindblowing it must have been to put in the PS2 disc for the first time back in '04. GTAIV, while undoubtedly vastly technically superior to San Andreas, is completely, COMPLETELY killed by its relationship system, lack of mission and locale variety (when I read about the original pitch for GTAIV, which  included the entire state of Liberty, I was heartbroken)  and checkpointing (I hear this has gotten fixed in TLAD and TBOGT, but I have not yet had the opportunity to play those titles). As for Vice City, I was not particularly enamored by the locale and the conjunction of many boat missions with a lack of a swimming mechanic annoyed me. III is interesting to me only as a museum piece, a "look how far we've come" comparison. 

I'm torn between Vice City and San Andreas. Vice City has an amazing soundtrack and setting but San Andreas has tons to do and is probably the only game of the GTA III gen that I've beaten.

VICE CITY

GTA 4

Vice City is close because of how great it does with the 80's atmosphere in all ways.

But San Andreas. Gameplay wise, the scale, RPG elements, CJ is a better character, and it does the 90's vibe very well in its own way. The whole first part of SA where you are deep in the gang aspect is the best GTA has ever been.

If you asked me to play one now, it'd be IV. Infinitely more playable and interesting gameplay wise. But I'm answering from when I was playing them on all on release and SA was the best it's been.

Saints Row The Third.

San Andreas is the "better" game but i prefer Vice City. IV is fantastic too.

IV is my favourite but I know I am in the minority, great story and characters, the driving and shooting is actually fun, The physics engine never gets old and the city is still a technical masterpiece.

@Video_Game_King said:

Saints Row The Third.

San Andreas with IV a close second.

Vice City is the one I have the best memories of. I don't know if it's the best but it's definitely my favorite.

Also I'd just like to add that Chinatown Wars is a fantastic game and it makes me sad that it didn't sell so well.

If I look at each game in a vacuum, it's a difficult choice. When GTA 3 was released, it blew my mind like never before. It was truly revolutionary. I enjoyed the 80's setting/soundtrack of Vice City the most. San Andreas was massive in terms of scope and gameplay mechanics. Gameplay wise, it's superior to its predecessors. The Boyz n the Hood/80's-early 90's LA gang vibe was pretty rad. It's tough, but I'd go with Vice City. GTA IV can go to hell.

@HumanoidTyphoon said:

The Boyz n the Hood/80's-early 90's LA gang vibe was pretty rad.

That's actually what I disliked about it the most. I like the gangster/mafioso trappings of GTA3 and Vice City (though VC's was filtered through 1980's Miami).

San Andreas was massive in terms of scope and gameplay mechanics.

That was the thing I liked about San Andreas. The RPG mechanics at work were really cool, as were all the character customization options.

I'd go with Vice City. GTA IV can go to hell.

This is the gist of my feelings.

I'd say the "best" would probably be San Andreas, but I liked Vice City way better.

Vice City is also the only GTA game to date that I completed 100%. I probably clocked at least 200 hours in that game. That's pretty much all I played for a full 2 years right after it came out. I got it it day one, too.

I went with Vice City. While San Andreas definitely has more GTA, I feel that Vice City is a tighter, more cohesive whole. Both games get points for awesome soundtracks.

I have played every one of them. I put Vice City at the top with GTA2 second. I play them both every year.

San Andreas. Vice City with a close second. When I go back to play an older GTA game I almost always go with San Andreas. It just had so much more to do and I loved the 90s west coast gangster theme.

Vice City for me, I know i played more of San Andreas but Vice City had style.

I'm actually going to say GTA IV just based on how much fun I've had on online Free Roam. Vice City is a close second.

Vice City.

Vice City was the one that nailed everything. San Andreas had more content, sure. But I felt saturated by it before the end. IV is impressive in dramatic terms, but there are things about the core gameplay that rocked the boat for a lot of people.

Vice City's setting and presentation puts it ahead of San Andreas, despite SA being a slightly better game. Personality counts for a lot in those games.

Vice City. The atmosphere and setting really put it over the top.

Well I've only played SA and 4 to completion, so I'll have to say San Andreas. I'm sure I'd really enjoy Vice City but I can't stand playing it on PS2 and I have countless issues trying to get it to run smoothly on my PC (The damn framerate is locked at 30)

San Andreas is just fantastic though. I hadn't played it back in 04 (because I was a kid), so last year was the first time I'd ever played it. I played through the entire game (on PC) to completion without having to resort to cheats. The fact that I was even able to enjoy it so many years after the fact with no nostalgia factoring into it says a lot about the game's quality. I gotta say though, if I didn't have the ability to get sick headshots 24/7 with those super accurate mouse controls I probably would have given up.

GTA IV because of the story...

Vice city was the last one i enjoyed and the one i enjoyed most (tied with london). could just not get in the san andreas setting and 4 was just a steaming pile of garbage ... how were you supposed to enjoy a game with bad driving, shoting and performance.

@Bourbon_Warrior said:

IV is my favourite but I know I am in the minority, great story and characters, the driving and shooting is actually fun, The physics engine never gets old and the city is still a technical masterpiece.

^. I get so sick of people who complain about the driving in 4. IV had the best driving physics of any open world game to be released before it, and any open world game to come out now.

I hear they're changing it in 5, and if it's shitty like saint row/sleepydawgs, I'm going to be mad that R* listened to a vocal minority.

As good as Vice City and San Andreas were, they stood on the shoulders of a giant. For that reason, I chose GTA III. IV can go fuck itself, I hate that game. And I've loved just about everything Rockstar has done but I couldn't stand IV at all.

Grand Theft Auto 2 - because it truly was the best GTA, they never clicked with me once they went 3D.

I really, and I mean really, love GTA 4. However, the fact stands that the GTA game that blew me away the most since I laid eyes on GTA 2 back in 2000, is San Andreas

Vice City had the best story and at the end of the day I'm a sucker for good narrative over gameplay.

Vice City is where it's at.

GTA IV for me. Mature, well written and acted, yet with alot of fun to do and better satire than previous games.

GTA San Andreas. Harrier....That is all.

Vice City

San Andreas wins in radio though. The talk shows and the commercials are hilarious.

Also the mods on GTA IV are so much fun.

@Bourbon_Warrior: Nice stuff, any tips on where to start with mods? I wanna go all-out with IV mods to play it once more before V but I dunno where or how to start. Thanks.

Kinda hard to say, Vice City had the best atmosphere and soundtrack and San Andreas had pretty much "everything" when it came to gameplay and stuff to do. GTA IV the best gameplay so far,controls and graphics, imo at least. But it has to go to Vice City, that game has a special place in my heart.

@OfficeGamer said:

@Bourbon_Warrior: Nice stuff, any tips on where to start with mods? I wanna go all-out with IV mods to play it once more before V but I dunno where or how to start. Thanks.

This should help you, links to mod sites are in the video description

For this gen GTA IV, for the previous gen GTA:Vice City

I adore IV. Not sure if it's my game of the generation but it's bloody close.