Halo is by far the best.
People don't say that so much because it was the debut game or because it's just so popular. It's popular because it is by far the best. For some reason that is beyond me, very few XBOX developers actually try to make good games that utilize all the powers of the XBOX, but Bungie (because they're owned by Microsoft and it was the debut game, and because Bungie is honestly a kick-ass company) really tried to make the best game they could in Halo. Why no one else makes such good games for XBOX is a mystery, because it's more powerful than ps2, which has most of the good games.
And to the people who say they were "dissapointed" with Halo or "thought it was a big let-down," I'd like to know exactly what you thought was wrong with it. If it has anything to do with A) the story, ur missing the big picture. B) the gameplay, ur a close-minded idiot. C) the graphics, ur blind. D) the two-weapon maximum, ur a pussy. E) the multiplayer, u need to play CTF on Blood Gulch. F) the marine AI, u have one measly good point.