Overall, Bioshock is probably the best gaming experience I've had. Mass Effect 2 and Red Dead Redemption were absolutely near the top as well. Love the Assassin's Creed franchise (especially 2 and Brotherhood) and if you like shooting things (
manga zuki me) you can't go wrong with Gears of War or Call of Duty titles. Some that haven't been mentioned, though, but are worth considering are is Batman: Arkham Asylum, Fable II, Oblivion, and the original Crackdown and Dead Rising.
What are your favourite Xbox 360 games?