If you haven't played it i strongly urge you to get Bioshock. You can get it for under a tenner now and it's well worth that.
Other games depend on your taste.
For FPS games then the Orange Box or Call Of Duty: Modern Warfare, Gears of War,
RPG's Mass Effect, Oblivion, or Lost Odyssey,
Sports then Fifa and Virtua Tennis 4 are the only two i really play,
Fighters, Street Fighter 4 and UFC are worth a look,
Action games, Ninja Blade is good fun, Devil May Cry as well. Both hack and slash but with a lot of fun for a bit of a release from the more taxing games.
And there are some great tiles on Arcade, Braid, Battlefield, Sam and Max and Monkey Island if you just fancy some classic pick up and play games.