Dead Rising. It's pretty cheap now. You won't get an experience quite like it on any other system.
While technically not an exclusive (it's on PS3 but that version sucks horribly), Bayonetta is worth looking into - I've only played the demo, but from what I saw it's on my must buy list.
Plants vs Zombies, again, technically not exclusive but the 360 version is by far the best.
Gears of War 2 is disappointing to someone used to FPS on a PC, although the story and visuals are pretty cool, the gameplay is lame and would work better with a mouse and keyboard.
Same deal with any of the Halo games, and no way Forge 2.0 compares with Garry's Mod.
Check out the Indie games, most of them are shit, but a few are quite good.
