It really depends on what you're looking for. Are you looking for a "sit down for 2 hours game", or a "sit down for 5 minutes" game?
PGR2 is a good "5 minutes" game, where you can play it for a few minutes, and then just take off and do your thing. It is also Xbox live enabled, which is a great feature.
KOTOR is a "sit down for 2 hours" game, very involved, incredible story, great gameplay.
For a fun arcade game, you can check out Crimson Skies, also fairly new, which is very fun, sit down for 5 minutes game.
People have been screaming praise for Prince of Persia, you can check that out too!
-Juice