What's the best steam game to play with friends?

Castle crashers, battle block theatre, rocket league, trine series, gauntlet

Killing Floor/ Payday

Mount your friends.

Rust can be very addictive if you play with friends

Borderlands 2, Serious Sam

Borderlands 2 is a lot of fun to play together.

If youre into it, Rocket League can be fun. Ive been playing that with some friends lately

Rocket League

Edit: He beat me to it

quack quack

Speedrunners.

ZOMBIE PANIC SOURCE NO MORE ROOM IN HELL planetside 2 evolve 2 FIREFALL warface MERCENARY KINGS IMPOSSIBLE CREATURES, STARBREAK, maybe hero siege, warframe airmech EYE DIVINE CYBERMANCY if you can figure out how to host with friends. its a great lan party game. evolution rts dont starve together insurgency defiance counterstrike pos CONTAGION (makers of zombie napic source and no more room in hell) brainbread 2? black light retribution mechwarrior online METAL ASSAULT ,RISK OF RAIN <- very hard to host , tree of savior, tera, RWBY <- very eh hack n slash game but great show, SANCTUM, SANCTUM 2 sven co op trove torchlight 2 ZMR, ORION <- assuming it still has this damn name.

Depends on your tastes.
But just to name a few on your game list that you've never touched:
Brutal Legend, Company of Heroes, How to Survive, Monaco

I second Company of Heroes if you like RTS games. Others I enjoy with friends would be Civ V and Sins of a Solar Empire. There are others we play, but most are old, and aren't on steam (Neverwinter Nights, SWAT 4, Freelancer, etc)

We also enjoyed Rainbow Six games, I think Rainbow Six 3 Gold is on Steam. If you enjoy tactical shooters and horribly annoying insta kill enemies.

[Superfight], believe me!! see the launch trailer at yutub^^ (just launched a couple days ago)
[Rocket league] if u r into that (which i am). [Payday 2] pretty good for summer sale price ($2 each for 4pack). [Dont starve together] are great. some ppl say [Battleblock Theater] , but dont have it, so i can say anything

Legend of Dungeon or
Hammerwatch

I'd also suggest Rocket League, works out great for 2-4/6/8 players and is tons of fun.
Dungeon Defenders II is a tower defense game thats also great with up to 4 friends, spent 2.2k hours in the first, and is free to play.
I see you've played Warframe, thats pretty good with a few friends though I am guessing you gave up on it. It is very grindy if you want to unlock everything but if you just want to experience a lot of the content and try the different frames its actually pretty good.
Golf With Your Friends is a nice casual game for several friends and is a good way to kill ~20 minutes per round.
The Borderlands series is always fun, especially as you are into FPS games.
One of my all time favorite games is System Shock 2 and it offers a coop mode, never tried it personally but still a good excuse to get the game.

Gmod or Dota 2

A couple friends and I started playing Magicka together tonight. We had a blast, but its very easy to get annoyed when your friends decide to blow you up. :3

Spintires

The only steam games I play with my friends are Dota 2 and CS GO

Rocket League anyone!!? You can trash talk your friend just for driving pass the ball. And trust me they'll miss. So much fun.

Age 2.

All of the voted list is fun to play with friends. Also Serious Sam campaigns/Survival. Evolve should get players more with F2P model. Skara is also awesome fun if you can find a crowd to slug it out. I personally find most satisfaction doing deathmatches in UT games.