The Best COOP game?

Payday 2 is really fun going on the hardest mode, portal 2 is fun, you can also play other games thats not coop thats not story driven, like garry's mod can be fun, also COD, I think MW2 Had a coop game mode like zombies but its MW so it's NPC enemys going after you, theres waves and at the end of the wave you can upgrade your weapons and the next wave goes