I play few multiplayer games, so I'm going to say Killing Floor. Saints Row 3 was alright as well.

Definitely Ignition. Loads of fun! :)

Ratchet and Clank: All 4 One was fun to play with my girlfriend. We tried Portal 2 but I think the puzzles required a bit more coordination with the controls than she was able to do.

A4O is goofy but entertaining. It's a lot less explosions everywhere than a regular Ratchet and Clank though, which is actually a bonus when playing with her.

I only played co-op on the NES. Battletoads, Ninja Turtles, Contra. Amazing.

I'm mainly a single player fan but when that urge for some sweet co-op comes around i love Borderlands 1 and 2 or Mass Effect 3...i'm going to purchase Diablo 3 sometime soon and it will be co-op madness on that as well.

doom II was a blast in coop, and when it worked, Unreal was also a coop gem. My best coop game by far though Dawn of War. A buddy and myself teamed up and steamrolled through a bunch of 2vs2 games. I still remember the map: Into the Breach.