Can you guys recommend some good CO-OP online games?

Squad or Post scriptum.

Why?/ there are NEED CO-OP lol.

The Forest. It is one of the best coop games I have played with friends. I guess most of you know about it. It's a survival game in which you need to build and escape an island (I won't say too much not to spoil).

Solo/Co-op up to 4 players zombie killing game (Dying Light)
Solo/CO-OP up to 4 players grinding/killing mobs, doing missions etc, Warframe (Free)

Garry's mod is very good game to play with friends

100% Garry's mod! It's my best game! I gaming in medieval server with my friends.

ARMA 3 is a brilliant game. So immersive and hours of fun to be had, especially with Zeus where you can create your own live missions

The Borderlands franchise, GTA is a lot of fun, Maybe Portal 2 if you haven't played that already..

Risk of Rain 2 is good fun with friends! Borderlands is definitely king though IMO

i always loved this game, Tom Clancy's Rainbow SiX Vegas , the co-op gives you more in-game interesting things

Heard from a friends Styx: Shards of Darkness is an awesome coop game!