Mass Effect 1 and 2 (and later this year, 3). Awesome games if you like rpg's at all.
Fallout 3 and Fallout: New Vegas are excellent rpg's, more of sandbox style games as well.
I really enjoyed Fable 3 too - again role playing game, more of a fantasy setting with some steampunk style in it.
lol did you miss the part that said COOP?