Best Steam Co-op game?

I've only played two games in Co-Op: Terraria and Battleblock Theater..

I'll just list some of the stuff I've played with friends recently that I enjoy.

Chivalry Medieval Warfare and Deadliest Warrior
Insurgency
Terraria & Starbound (even though I hate the development team of Starbound)
Payday 2 (even though it is a DLC fest)
Nosgoth
Tabletop Simulator
Endless Legend
Oooh I almost forgot Divinity: Original Sin

I was also gifted Don't Starve Together Beta so I'll be trying that out after exams. I enjoyed the single player.

Super Mario Bros 3

Insurgency and Sven Co-op are usually my go to

Wouldn't know, I have no friends. But I really enjoyed Portal 2.

Splinter cell Blacklist

Me and my friend really enjoyed Borderlands 2 co-op. Trying to get him to get the pre-sequel now since I just got it.
Dungeon Defenders is also really fun with friends, I just got it and if you haven't played it I suggest you check it out.

Payday 2 with or without DLCs.

Mount&Blade:Warband

• Risk of Rain
• Monaco

Magicka
Monaco
Battleblock Theater
Don't Starve Together
Torchlight 2
Portal 2
Sanctum 2
Payday 2
Borderlands 2

if you have massive amounts of time and you like strategy, try Ai War: Fleet command

1.- GTA V online ( wait 4 launch )
2.- Left 4 Dead 2
3.- TF 2
4- Metal Slug
5.- BF 4
6.- Battle Block Theater
7.- Heroes 3 HD edition
8.- Sacred ( any )
9.- Two Worlds
10.- Borderlands 2

Portal 2 is really good. A bit short, but the workshop can help. Terraria is good, and Borderlands 2 is awesome.

Battleblock Theater
Mount Your Friends
Goat Simulator
Sonic 2