Left 4 Dead is gonna be a co-op breakthrough in the future. Tales of vesperia will have fun co-op, and so will gears 2 and Fable II. Too Human has co-op as well.
Oh wait...do you mean already released? Geometry Wars 2 is pretty good; so is alien hominid, Doom, and 1942.
GOTY Contestants this year: Dead Space 2, Dark Souls, Tales of Graces f. Everything else can suck it.