Resident Evil 5

This is probably the best co-op game I've ever played. Online is still active, especially with players looking for teammates every day, so it's easy to find a friend to play with. The controls might be a bit off-putting for some, but trust me, you'll get used to them after a few hours. What I love about this game is the interaction with your partner. My god, it's so enjoyable: sharing and receiving ammo, healing your friend, the teamwork. I recommend everyone play this game, but only with a friend or partner
As for the plot? It's probably ordinary, there's some surprise, but it's such a cliché
At the moment I haven't completed this game to platinum yet, but I will slowly work towards that mark