Portal 2, L4D2, Borderlands 2, Torchlight 2. Must have well over 100 hours on BL2, just for the co-op alone.
One issue I have with L4D2 is the bots.not that they are bad, but they distract from the experience. Really is a 4 player game.
To be fair, most of the other games are better with a couple more friends. Co-op, 3-4 players, that's the sweet spot.
And yeah, Deathwing looks promising.