Rivals Broken Steel
This is hands down the best DLC for New Vegas, and even rivals Broken Steel as maybe the best Fallout DLC yet. Simply put, it does everything right that a Fallout DLC should do.
It gives you a decent sized new open world to explore that is quite varied, and includes a lot of areas you can go inside to explore. That is exactly the kind of exploration layout that a Fallout game should use.
The story is also excellent, it really has a mid 60s sci-fi feel to it. The characters are interesting and entertaining, and I think it has the best dialog found in New Vegas. I laughed on quite a few occasions. It even has a long ending cut scene that is segmented based on your actions and choices just like the end to New Vegas itself, and it is quite detailed and long (almost as long as the ending to the main game). It really makes you feel like your actions mattered.
There are of course some new weapons and armor, and while they are nothing ground breaking they are actually good enough to use in your main arsenal unlike most of the weapons that get added. Plenty of new perks and skill bonuses as well. Best of all, you end up with a new "house" at the end that is loaded with everything you could ever need (work benches, storage, healing, vendor, and a slew of other new things) that is also super convenient to get to.
As a bonus it even ties into the story of Dead Money a little, and explains a lot of the backstory about what happened at the Sierra Madre to setup the disaster there. From the Ghosts to the holograms and even the vending machines. Even a lot of the characters themselves from Dead Money have left footprints in the Big MT. There is also a setup for the next DLC Lonesome Road.
I really don't understand what happened between this DLC and the first two for New Vegas (Dead Money and Honest Hearts), but words simply cannot describe how much better this one is. I didn't hate the first two, but they were not that great either. This thing simply blows them out of the water. I would love to know what happened behind the scenes that made this one 100x better.
If you own New Vegas, you must play this.