I would have to say that Resident Evil 5 has been my overall favourite co-op title thus far. It was not without some faults, but it is definitely the one co-op game all other since have been measured by for me.
My friend and i played nothing else for over a month solid, we just kept replaying it and trying different difficulty levels and kept going for higher scores...sheesh, even after eventually selling it off i've owned it 3 more times! (it came with the red 360, eventually traded that copy and bough it used when the DLC came out, eventually traded it and then saw it cheap last month and bought it again...I may have a problem...)
Nothing has come close to the gameplay and actual co-op feeling of the experience...although I have to admit, after playing the DLC in the mansion, I wish they would make a long older style co-op game like that!
Actually, that would be my major complaint, that the game is far shorter than it should be...although I guess when you are having a blast with a good game it can always feel that way...just after playing Res 4 and clocking in around 20 hours of gameplay it seemed odd to blast through in around 8 hours on Res 5.
Oh well, still love the game!
EDIT: I decided to add some honourable mentions here as well.
-Borderlands
-Army of Two: 40th Day (only fun in co-op)
-EDF: 2017 (cheesy, but i think that's the point)
-EDF: IA (it's at a bargain price and just fun to play)
There's probably many more I have forgotten, but this will do for now :)