I would say WoW but i think thats kinda cheating. So id have to say halo 3 only if im allowed to play online. Halo 3's online is by far my favorite online out there. The game came out over 5 years ago and i still get on every once and awhile.
Well i personally think that fallout is a better overall experience. Except for once you beat it and beat all the missions the only thing left to do is roam around and kill things or start over. Borderlands definitely provides you with a lot more entertainment. But just general being fun i think that fallout definitely wins. Nothing is cooler than blowing someone's head off with V.A.T.S