Best CO-OP Games?

Amnesia.

1 player on the keyboard, 1 player on the mouse. Thank me later.

Magicka, Sanctum, Really Big Sky (it's for one pc and I didn't play it, but I think it will be as fun in coop as it is in singleplayer), Dead Island, Frozen Synapse.

Dungeon Defenders, L4D, L4D2, Portal 2, Payday The Heist, Beat Hazard, Dota 2, LoL, HoN, Magicka, TF2, Trine, Trine 2. :)

I had a lot of fun with lost planet 2 co op. super fun killing giant enemy's with your friends.

Borderlands 1 and 2

L4D and L4D2

Left4Dead series
Borderlands series
Magicka
Torchlight 2
Alien Swarm
Trine series

my favs

Borderlands 2 is great! I've also played some magicka co-op. Awesome you can just blow up your friend revive him again and then push him off a cliff before he has the chance to do anything back and then revive again without losing anything (like mana or lives).

Besides the ones already mentioned, Gundemonium series, if you're both into shmups.

Resident Evil 5

Terraria

Sanctum

portal 2

Left 4 Dead 1-2 /
Terraria /
Minecraft /
Portal 2 /
Dead Rising 2 /
Dead Rising 2 Off the record. /
Killing Floor /
PAYDAY: The Heist /
Dungeon Defenders /
Bloody Trapland /
And many more.

Borderlands 1. Haven't played 2, so can't attest to that yet.
Left 4 Dead 1 and 2. So long as people can work as a group, you're golden.
Dead Island. Definitely better co-op than single player.
Saints Row 2 and 3. Nothing like tearing through a city with a partner in crime.
Portal 2. My friend and I think in different ways, which made tackling puzzles easier.

Torchlight 2, Trine 2, Dead Island, Resident Evil