My favorite Co-op experience has been tackling the enemy AI in the co-op campaign of GRAW 2. Using the meet-up thread here, every Friday night was just fantastic.
We'd start the match, choose our loadouts, and talk strategy as far as what to expect from the enemy and map's layout. Aside from the fact that the maps never change in reagrds to how the enemy fights. It was a great experience to have 8 people on and devise ways to achieve victory.
I love co-op because it allows me to play games I love with other people who enjoy them as much as I do.
My favorite co-op experience has been playing Gears 2 (particularly Horde mode) with my friends from here and Colony of Gamers. Beating wave 50 was a fun achievement, but it's the specific moments leading up to the win that made it enjoyable for me. It's always exciting when your teammates are down, and it's up to you to revive them while somehow battling off what seems like an unstoppable onslaught of Locusts. Or the frantic moments where you rush over to save someone from a Mauler, only to be downed, but then someone else saves you and gets downed, and this keeps on happening (all while some jerk Mauler is swinging his mace at everyone) but you miraculously all come out alive and beat the wave. (Phew! Long sentence!) I'd have to say that those are the most exciting moments for me.