Well. I bet most people are going to say Halo. But...I get to be the first! :woot
So my favorite XBOX game is Halo. There's a lot of things I like about it.
-It actually has a story, especially if you read the books that go along with it
-There are lots of weapons
-Vehicles! The vehicles rock. I like the tank and the Warthog especially
-Multiplayer goodness. Co-op, split screen, system link, all of them rock! My favorite of the moment is Free For All Slayer with Rocket Launchers only.
-Even when you play with the same people in multiplayer, each game is different
-It inspired the best web series ever: Red Vs. Blue. All hail Rooster Teeth Productions!
Another favorite is Amped 2
-It's not as campy as the SSX series
-The controls are much improved over the first Amped
-Good soundtrack
-Neat career mode
I also enjoy Baldur's Gate: Dark Alliance 2 and D&D: Heroes.