I enjoyed Army of Two with a friend (the first and the second). RE5 is good if you and your partner actually know how to play (you know, heal each other, share ammo / divide guns, not shoot EVERYTHING and melee when you can). Then there's Rockband / Guitar Hero, but those are usually more fun in-house rather than online. Dead Rising 2 wasn't so bad, but it's also not the BEST co-op experience. Tekken 6's online scenario is decently fun if you like Tekken games, but again, not exactly the best co-op game out there.