Gears of War 3 should easily take this when all is said and done but there's been plenty of awesome co-op and more to come.
Uncharted 3
LittleBigPlanet 2
Call of Duty Modern Warfare 3
Portal 2
Ratchet and Clank All 4 One
and plenty of others. Basically if your a gamer who enjoys co-op this year will have lots to keep you satisfied.