None of the Halo's are really that great of games. They just happen to have really fun multiplayer. Go figure.
I mean, correct me if I'm wrong, but Halo never has, and probably never will hold a place in the "best of 200x" categories because, well, all around it's kind of a mediocre game. Solid, sure, though single player may be a bit lacking, but not mind-blowing or revolutionary like, say, Bioshock or Dead Rising.
It's good for what it is, a mindless, no strategy involved, run up and kill things like rambo game.