Co-op games are a rare breed in the current climate. Most thrive on a "You against the world" mentality, and even online team modes feel like a loosely assembled set of individuals rather than a genuine unit. When a great co-op game comes along though, it can leave a profound impact.
Your reliance on another player, plus the awareness that they're relying on you, completely changes the gameplay experience.
Where online team games largely see you heading out on your own (albeit potentially adhering to tactics), co-op titles merge two players into one. Often, you need to be completely aware of what your partner is doing, all while doing your own thing to support them.
Because of this symbiotic nature, a lot of the games featured here are puzzlers. Problem solving and figuring out who needs to do what (and where they need to do it) is a great way to get duos working as a unit. Others are solo-esque games (some are even playable entirely in solo), which ask co-op players to come up with complementary playstyles.
These are the games which can make or break a friendship.