Great coop game for me and a friend to play?

I had just introduced my female friend to Steam and gaming last few months and she really liked it, we loved Portal 2 but we are looking for another great coop game but we can't find a good one, I am pretty sure some of you here knows a great coop game?



Here are (so far) that we had played: L4D2, Portal 2.

Great coop game for me and a friend to play?