Just the full restoration mod (it also makes the game playable on modern PCs). You can find it in one of the pinned threads on the game, that's where I found it.
As for other mods, I'm not even sure. The game is easily 40+ hours vanilla without doing 100% of the quests. Then the DLCs are probably about 5ish hours each (some are better than others, the DLC is definitely not New Vegas' strong suits) and they come with the Steam GotY version so it's not so much a waste.
I know there's a huge Fallout Mod community (I've got a bunch for 1 and 2 that literally add a whole new dynamic and in 2's case, another 10ish hours of gameplay) but I still haven't even attempted to side with one of the factions in its vanilla version, which changes the game quite a bit each time.
It's so much fun. It's a game where your character achieves a notoriety which makes certain people aggro you instantly or on the other side of the coin, gets you benefits with a different faction. God, I love this game so much.
The restoration mod (not even sure if it's so much a mod as it is a giant patch) is a definite MUST, though... your game will crash constantly without it. (again, it's in one of the sticky threads on the Steam game board).