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

You can never go wrong with a L4D2 game. Even today, it holds up with alot of cooperative play. And mod the game together,

Destiny 2 is fun as all hell If you like that kind of genre of games.

Left 4 dead 1 and left 4 dead 2 portal 2. portal 1. garry's mod.

If you're a CO-OP player, try a way out. It's honestly a good game.

you can try Borderlands 2, Don't Starve Together, Garry's Mod, Payday 2, and Portal 2

Okay I will give you a list of really great coop games in my opinion. Garrys Mod, Dead Island, Dead Island Riptide, Dying Light, DayZ, Arma 3, Ark Survival, Payday, Borderlands 2, Raft, 7 Days To Die, TruckersMP(Euro Truck 2/ American Truck Simulator), Just Cause 3, MTA SA, SAMP, Squad, Storm Works(Requires very High End PC) and Grand Theft Auto V. Here are all the COOP/Multiplayer games I could think of. There are some more but cant remember. Hope this could help you.

Balloons tower defense 6. I waste way to much time on that game.

The new Modern Warfare Coop looks pretty damn good.

Deceit and Dying Light are epic. The Borderlands franchise is also good.

Garry's mod, not exactly a co-op, but you can have fun