What Steam Game Have You Spent The Most Hours Playing?

DD 128hrs

Deus Ex: Human Revolution - 48.6 hrs on record

But a lot of that time was alt+tabbed doing something else.

The CS: Source playtime is all from the hacker that had my acct for a while.

Dungeon Defenders with 108.6 hours

"dota 2" 2673 hours

Definitelly it's TF2, but you know, almost everyone idles, so... Gmod wins.

Modern warfare 2 622 Hrs
Team fortress 2 166 Hrs

ARMA II: Operation Arrowhead: DayZ Mod. 300+ hours logged, and that's only from when I was played with Steam enabled. I'd say 500+ hours and i haven't even had it 6 months.

Team Fortress 2-249.

Garrys Mod-106

The Hidden-Around 60-70 H.

How can people get that much hours, like 2000 or so?

CS:S

Dota2 175Hrs

It may seems strange but Mass Effect. Over 330 hours.

TF2 for a whopping 844 hours. Next comes surpisingly Terraria with about 250 hours. Not bad for a 2.50€ game :D
http://www.lambentstew.com/webblog/miniproject/steamanalysis/steam_time/ISAgamers

Team Fortress 2, even though i have 1612 hours, 600 of those are like for idling.

Counter Strike: Source - 453 hours ... Borderlands - 114 hours... and few some about 40-50 hours ;)

Super Monday Night Combat, 806 hours played. But waiting on matches and stuff can bring the time down. I have at least 20 days in solid gameplay hours (based on the protag tracker in game).

Notable other games:

287 hours on Vindictus (I actually played it before it was on steam).

156 hours on Skyrim.

73 hours on Borderlands 1.

Team Fortress 2. Played a ton of Tribes: Ascend and Planetside 2 in the betas.

It doesn't show up, but I played 1000+ hours in Half Life 2: Deathmatch. Unfortunately, it was all before steam started tracking game time.

MW3 618 hours altho it's a crap -.-'

Battlefield 3 - 229h
Borderlands 2 - about 216h
These are the most played games I have registered
I like to play many games so I don't have a big amount of hours on one single

Day of Defeat: Source
2737 hours played.

Time to go and play some more.

TF2 2:023H
dota2 7:24H
Killing floor 5:29H

Dota 2 853 hours

Dungeon Defenders - 177 hours.

Borderlands 2 likely to pass it before I'm done the game completely. (currently at 127 hours)