Gears of War. Hands down. I have replayed it so many times now I know it off by heart. Also the multiplayer, ooohhhhh the multiplayer, It's awsome on a stick. Oh yeah, and Half-life, either of them, and the Thief series, oh and System Shock 2, oh oh and Bioshock, wait...and...and...No thats it really.
I honestly have never plated Gear of War.