The Gears of War games are pretty fun, though I don't like the MP to much.
If you want a deep 3D fighter check out Soul Caliber 4 it's an absolute blast.
Assassins Creed 2 is awesome (If you care about the story you can get the first one I suppose but having played both, I'd just skip it).
The Crackdown games are pretty fun if you like the whole open world GTA feel with superpowers.
Farcry Predator Instinct was a pretty decent 360 rendition (note not port, it is its own game) of the series. Don't get Farcry 2 though, sucked =/
Mirrors Edge is a really fun "parkour" game. Just running along the rooftops and leaping across city streets is a exhilarating experience unmatched in any game.
For racers the Forza series is probably the best of any console game out there.
If you want a brutally hard but awesome game, check out Ninja Gaiden 2
. Once you get the basics of combat it's one of the funnest hack n slash action games in existence. If Ninja Gaiden isn't your thing because of the difficulty Devil May Cry 4 offers similar gameplay. If you want to go WAY over the top, check out Bayonetta.