What Steam Game Have You Spent The Most Hours Playing?

I'm slightly ashamed to admit, but Dota 2.. XD

TF2 idling, when I did care about hats.

cs 1.6. 1100+ hours

Team Fortress 2 at 317 hours, followed by Battlefield: Bad Company 2 at 104 and Terarria at 95.

Non-Steam I have 183 hours of Diablo 3, and I don't even know how many of League of Legends. It says 1,409, but that includes client time, and I sometimes leave that open. Definitely a lot of League, though.

650 Hours on Garry's Mod

Defense Grid, 297 hours and I still haven't done all the maps!

Warhammer® 40,000™: Dawn of War® II - Chaos Rising™ 447.8 hrs on record

Terraria - near 300 hours

TF2-503 Hours since it became F2P.

The Elder Scrolls V: Skyrim, by way too many hours for having never completed the game. I spend way too much time starting over.

Well, I suppose it depends on what you mean.

Most hours in a game/franchise: Counter-Strike (CGO - 20hrs, CSS - 279hrs, CS 1.6 - 1700+hrs)

Most hours (recorded) in a single game: Left 4 Dead 2 (1055hrs)

Most hours in a single session: Mass Effect (18 hours)

The most hours I've ever spent in ANY game is Diablo II, a non-Steam game. I've spent over 2500 hours in that game and I still enjoy it.

Borderlands, something like 86 I think?
Can't check right now, still playing though, want 100% but I'm going to beat Borderlands 2 first.

Civ.

cs

Terraria - Got 231 hours on that...

...My God I'm wasting my life...

ON AWESOME GAMES!

Counter Strike ~1400 hours

Team Fortress 2 416 hours. it was fun to play BEFORE the hats and workshops.Its still fun though

Dota 2...around 300 hrs.

X2: The Threat - 413 hrs

The Basement Collection - 52.2hrs XD

OnSteam: Civ V - 40ish hours
OffSteam: Diablo 3 - 300+ hours
Warcraft 3 - ~8 years of active online playing

Team Fortress 2 -> 1,146 hours (including idling :D )
Dota 2 -> 558 hours

Team Fortress 2, too many though some were idle, then Skyrim, then Borderlands.

Borderlands, about 250 hours.

I spent 167 hours on Football Manager 2012.

That's about 12 solid days of my life :-O