Which Steam game have you got the most play time on?

L4D2 - 1001 hours.
EU3 (Europa Universalis III) - 346 hours.
TF2 - 79 hours.

With this account Borderlands I believe with Audiosurf close behind.

TF2 also.

Alien Swarm
1.6 ч.
Brink
4.6 ч.
Call of Duty: Black Ops - Multiplayer
1008.8 ч.
Call of Duty: Modern Warfare 2 - Multiplayer
1024.8 ч.
Counter-Strike
8.4 ч
America's Army 3
0.2 ч.
And Yet It Moves
0.1 ч.
Call of Duty: Modern Warfare 2
16.2 ч.
Call of Duty: Black Ops
9.6 ч.
Crayon Physics Deluxe
1.7 ч.
Half-Life 2: Deathmatch
2.6 ч
Left 4 Dead
2016.2 ч.
Left 4 Dead 2
157.1 ч.
Team Fortress 2
1.7 ч.

Half-life² DM

Team Fortress 2 D:

You can always check "Wasted on Steam dot com" to have some laughs.

Here is mine: http://wastedonsteam.com/usd/id/echomateria.

I feel so inexperienced with my meager 138 hours into TF2 (which is my top game).

Team Fortress 2, about 570 hours of actual gameplay. I have nearly 900 hours otherwise but I've never idled. I guess the load times and AFK breaks really add up.

Counter-Strike: Source followed by TF2 :P

over 500 hours of killing floor <3

Black Ops, about 200 hours

Team Fortress 2 with 784.7 hours and i never idled.

L4D2 :)

As of now, Team Fortress 2 with 540 hours under my belt.

Team Fortress 2
1991 hours played

Problems

MW2 over 1k hours ftw ^^

Counter strike source with 1320 hour , but it's my second account and iwas nearly 1000 on my other

So it's +- 2320 hour on css

Then 350 on tf2 and 190 on HAMMER

Call of Duty: Modern Warfare 2 - Multiplayer 613.4 hrs on record. I am thinking TF2 will soon overtake.

L4D2
177.7

Killing Floor with 117 hours, but TF2 rapidly rising :D

Counterstrike 1.6

253 Hours on Empire Total War, thats my most played game on steam, though I'm pretty sure if I could look at my hours for non-steam games Europa Barbarorum or Mount and Blade probably have more hours, or Eve Online from when I used to play that.

Team Fortress 2, 91.2 hrs / 402.1 hrs, joined at June 26, bought premium Team Fortress 2 the same day.

Team Fortress 2 by a lot!