My favourite always been Resident Evil series.
RE5, RE6, RE Revelations 2.
I've played Knack 1 and 2 as co-op as well. Equally satisfying. Lego a lot of fun, UNO has co-op mode too.
How To Survive 2 is darn fun to play as 2 players, Sky Force Anniversary is an arcade classic coop. Rayman Legends is similar to little big planet, can play up to 4 players.