Tell me the Game on which you have spent the most hours on steam. xD

Mass Effect. 339.4 hrs on record. (link)

Garry's Mod, 617 hours.

Crimecraft gang wars : 456h
M&B WB : 179h (with probibly 400 played without steam edition)
DC universe online: 159h
TF2: 149h
Global agenda: 137h
Empire total war : 96h
The Secret World : 90h
i got a few more but wont list em all

Dungeons of Dredmor - 138 hours
I really love this game.

3,098.0 hours on Dungeon Defenders and 1,000 hours on Snuggle Truck.

Played:
72.3h - Civilization V
70.7h - Tropico 4

Just idling:
792.2 - Forsaken World
255.2 - Team Fortress 2

Borderlands 2 62 hours and counting

I'd like to add that neither TF2 or The Binding of Isaac is my favorite game. They just happen to be games that are easy to pour many hours into. My favorite games would be Super Meat Boy (95 hours), Cave Story+ (59 hours) and Machinarium (9 hours). Of the games I'm currently playing, FTL probably has the most potential to go over 100 hours.

Garry's Mod - 527 hours

Listing: L4D (1035h) TF2 (601h) DOD: Source (530h) L4D2 (480h) KF (400h) ... playtime in 2 weeks (between 280 and 300 hours) and i have no life

Played: Cs 1.6 2857 hours

Idle: Universe sandbox 8507 hours

Terraria, but Borderlands 2 might overtake it when I'm going to play all the campaign DLCs from the Season pass I got

Skyrim - 394
Borderlands 2 - 254
Killing Floor - 211

Garry's Mod with around 670 hours? That sandbox mode is seriously addicting and so are the roleplay gamemodes D:

Payday: The Heist 301 hours

Counter Strike 1.6 with +1000hrs D:

Dungeons of Dredmor, 335 hours.

Call of Duty: Modern Warfare 3 - 118 h (both multi and single)

Counter Strike: Source,got it 1-2 months ago,over 200 hours played

Team fortress 2 with 605 hours <_<

Around 200 hours in Dota 2. I have 500+ in TF2, but around 470 are pure idling.

Terraria, wth 144 hours. I lost all of my files because i got a new laptop and forgot to transfer my steam folder to my portable HDD. Recently started again because of the possible update.

Left 4 Dead 2 - 1500+ hours, mostly Expert Realism w/ friends. Nowadays it's on add-on campaigns for the most part.

Now, Terraria. This weekend a friend in one of my Steam groups decided to start a server for us, since then I played around 12 hours straight one night and over 8 hours the other. Sad part is for the 8 hour time I spent it mostly changing our homebase from being made of wood to being made of copper brick and organizing our treasure chests.