#1 Ace Attorney Investigations: Miles Edgeworth
#2 Metal Gear Solid: Peace Walker
Surprised Peace Walker didn't make the list. Sure it was on handheld but it was such an amazing game with interesting mechanics.
#3 Lost Planet 2
This game was so much fun but there was a level pretty early on that required a good team play which I sometimes failed because someone got kicked out and made it frustrating to get through. It had some serious hidden stuff that really elevated the game if you knew how to do it. Also taught my cousin a skill that got him a world record at the time on one of time attack stages but has probably been beaten since you can cheat on it somehow.
This is probably one of my favorite co-op game simply because there is not many like it.
Gran Turismo 5
Didn't play enough of it but I always loved GT games
Valkyria Chronicles II
Co-op lets you basically have an extra character so it was pretty easy but soloing was fine too. I loved the classes that split to pick which speciality to go for so it was really fun. Escort missions were the big minus but I didn't mind the cliché high school setting outside of awful laugh from protagonist Avan.
This was a really good game and I like some music from that era. Big fan of Dean Martin. Sadly lack of side stuff and freedom puts it behind other stuff