Best Co-op/MP Game on Steam?

-----> Garrysmod <-----

This is not on steam though, but another one I would suggest is Minecraft.
A way better version of Terraria.

left4dead, l4d2, black ops, mw, mw2, cs , css, tf2, minecraft, bfbc2, portal 2

Borderlands, l4d ofc.

Left 4 Dead 1-2, Counter-Strike Source-1.6,

Terraria and Borderlands, the best and cheapest choices.

while not steam, minecraft is great.
also you can download it from any computer by logging into your account so.. it sort of has that steam-ish functionality.

There's no definite answer for that. Furthermore, that's only something YOU yourself can decide. We can only give you games you can try out and see if it helps you decide.

That being said, I think you should give "Left 4 Dead 1/2, Team Fortress 2, Magicka (if you can get it to run properly), Sanctum, and/or Borderlands" a try.

Terraria, L4D or Killing Floor :3

DoW2

Killing Floor nuff said.

Nation Red is a good zombie mashing game for up to 4 players, it really is rather manic.
Most RTS games. Play VS AI, Play VS other Humans as a team with your buddy
Company of heroes is a good choice.
Fable 3, some people love it, some hate it. Coop always makes a game enjoyable!
Guild wars + Expansions, currently playing through this with my friend, just the two of us no other publics, having a riot of a time!
Men of War, seriously, so stupid things happen makes this hardcore RTS worth trying with buddies.
Saints Row, or Lol as my friends call it, 2. Excellent Coop, full of laughs and gags with lots of content to do!
Splinter Cell Conviction. Large amounts of Coop content, more fun if you imply balls to the walls rules for your self.
Titan Quest, Diablo clone of excellent quality!

Not on steam but easy to get from D2D.
Swat 4. TRAILERS