For me, personally -

1 -

Team Fortress 2

106.9 hrs on record

2 -

Counter-Strike: Source

98.4 hrs on record

3 -

Fallout 3 - Game of the Year Edition

63.5 hrs on record

4-

Hitman: Blood Money

33.2 hrs on record

5 -

Garry's Mod

27.2 hrs on record

What about you guys?

TF2, L4D2, Stalker: Call of Pripyat, ME2, plants vs. zombies

I cant say, as for some reason it records the times for some of the games, but not all.  For example: it says I've only played Fallout 3 for 3 hours, which is most definitely not correct.

1. Team Fortress 2 - 92 hours.
2. Audiosurf - 55 hours.
3. Lumines - 49 hours.
4. Altitude - 40 hours.
5. Borderlands - 39 hours.

1.  Left 4 Dead  2.  Borderlands  3.  Garry's Mod  4.  Counter-Strike: Source

1. Team Fortress 2- 147 hours.
2. Garry's Mod- 76 hours.
3. Tropico 3- Steam Special Edition- 25 hours.
4. Audiosurf- 22 hours.
5. Warhammer 40,000: Dawn of War II- 21 hours.

Borderlands
KOTOR
Mass Effect 1 & Mass Effect 2
Plants Vs Zombies

1. MW2 Multiplayer -  238 Hours. 
2. Team Fortress 2 - 210 hours. 
3. Counter Strike Source - 145 hours. 
4.  BFBC2 - 117 hours. 
5. GTA IV - 204 hours.  (About half of them were the Rockstar Social Club running while I wasn't even playing the game)

Borderlands 105.2hrs
Mass Effect 67hrs
Mass Effect 2 (64)hrs
Torchlight 52.1
and......*drumroll*
Star Wars: Knights of the Old Republic 44hrs.

Team Fortress 2  84 Hours
Audiosurf  12 Hours
Splinter Cell Chaos Theory 10 Hours
Garry's Mod 9 Hours
Portal 6 Hours

• Team Fortress 2 - 127 hrs

• TrackMania United - 100.1 hrs

• Trials 2: Second Edition - 19.2 hrs 
• Saints Row 2 - 13.7 hrs
  
• Call of Duty: Modern Warfare 2  Multiplayer - 11.7 hrs

"

• Team Fortress 2 127 hrs

"

Counter Strike: Source 870 hours and counting but i have probably played a total of 2000+ because i have uninstalled numerous times. 
Then Civ 4 and 5
 Mass effect 1 and 2
Half life

Civ 5 and Torchlight

• Dawn of war 2 -  378 hours
• Dawn of war 2 chaos rising - 176 hours
• Fallout 3 - 99 hours
  
• left 4 dead - 69 hours
• left 4 dead 2 - 62 hours

" @MattyFTM said:

"

• Team Fortress 2 127 hrs

"

I had to read these 4-5 times before I figured out that you hadn't played Team Fortress for 2127 hours. "

1. Mass Effect - 111 hours
2. Flotilla - 87 hours played (Wait, what? Wow. That's gotta be a record for that game.)
3. Star Wars: Knights of the Old Republic - 79 hours
4. Dragon Age - 72 hours
5. Mass Effect 2 - 71 hours

410 hours in TF2 since they started counting hours played. Easily had 200-300 before that. (And none of that is idling, jerks.)
 
 
I have far too many games to go through and find the rest of my top five.

tf2 -245 hrs  
css- 220  hrs
cod4- 68 hrs  
Grand Theft Auto IV-29 hrs 
Age of Chivalry -29  

I've had my times reset multiple times, so I wouldn't be able to say accurately. Besides, the largest portion of my playtime came before the Steam overhaul where it would actually accurately count your time played.

Counterstrike 285 hours 
TF2 57 Hours 
Battfield BC 2 55 Hours  
MW2 47 Hours
L4D 42 Hours 
 
Honorable mention of .8 hours of Floras Fruit Farm... nothing like singing to trees so they grow. 

I haven't been on steam for very long, so my time played is pretty meh right now.
 

1. F1 2010 - 29 Hours
2. TrackMania Nations Forever - 20 Hours
3. Left 4 Dead 2 - 11 Hours
4. Left 4 Dead - 6 Hours
5. Borderlands - 6 hours

1. Civlization IV 76.4 hours 
2. Civilization IV: Beyond the Sword 54.8 hours 
3. Left 4 Dead 2 68.6 hours  
4. Team Fortress 2 62.7 hours
5. Left 4 Dead 60.1 hours

The Elder Scrolls IV: Oblivion - 14.6 hours

Borderlands - 14.4 hours

Left 4 Dead - 11.4 hours

Freedom Force vs. the 3rd Reich - 9.6 hours

X-COM: UFO Defense - 7.1 hours

Steam also claims I've played Dark Messiah Multi-player for more than 24 hours, when I've never even been able to launch the game, I'm pretty sure it didn't notice that the game crashed when I tried to start it, and then logged all the hours I spent in Oblivion for that game instead, since I've played Oblivion a lot more than 14 hours (it said 17 or 18 hours before a time reset a couple of months ago, so the 14 hours it displays are in addition to those, and the Dark Messiah 24 hours are probably also all Oblivion time. I've also spent a lot of time with Fallout 3, but the Steam time stats only says 5 hours, that's because it for a while only logged the time I spent looking at the launcher (the program where you can select which mods and options to run the game with), and not after I started the actual game.

• Team Fortress 2 - 127 hrs

• TrackMania United - 100.1 hrs

• Trials 2: Second Edition - 19.2 hrs 
• Saints Row 2 - 13.7 hrs
  
• Call of Duty: Modern Warfare 2  Multiplayer - 11.7 hrs

 
It should be noted that I often play single player games in offline mode on steam, so not all my hours playing games on steam are counted.

1) Borderlands (134 hours)

2) Call of Duty: Modern Warfare 2 Multiplayer (94 hours)

3) Mass Effect (85 hours)

4) Mass Effect 2 (74 hours)

I dunno how the hell I did this.

1) Team Fortress 2 -  119 hours played  
2) Left 4 Dead 2 - 28 hours played  
3) Civilization V - 26 hours played  
4) Warhammer 40k Dawn of War II - 10 hours played  
5) Plants vs Zombies -  5 hours played  
 
However, this does not include games which are not available via steam like Starcraft II.  You don't even want to know my /played for my WoW toon.

My stats on some games were reset at some point, not sure why (maybe due to OS reinstalls?), so this is in no particular order: 
Team Fortress 2 
Counter Strike: Source 
HL2: Deathmatch 
Day of Defeat: Source 
GTA: San Andreas

1) Team Fortress 2 -  146 hours
 
2)Knights of The Old Republic - 48 hours
 
3)Recettear - 9 hours
 
4)Half Life 2 - 9 hours
 
5)Shatter - 7 hours

1) Team Fortress 2 -  119 hours played  
2) Left 4 Dead 2 - 28 hours played  
3) Civilization V - 26 hours played  
4) Warhammer 40k Dawn of War II - 10 hours played  
5) Plants vs Zombies -  5 hours played  
 
However, this does not include games which are not available via steam like Starcraft II.  You don't even want to know my /played for my WoW toon.

1.  Plants Vs. Zombies (163 Hours)
2. TF2 (111 Hours)
3.  Warhammer 40k Dawn of War II (51 hours)
4.   C&C Red Alert 3 (51 hours)
5 . Civ IV (29 hours)
 But Like others mentioned it doesn't seem to be tracking well. I built the computer before the one I'm on for the release of Half-Life 2 and it says I've played 3 hours and I've never changed steam accounts.

" @MattyFTM

• Team Fortress 2 - 127 hrs

• TrackMania United - 100.1 hrs

• Trials 2: Second Edition - 19.2 hrs 
• Saints Row 2 - 13.7 hrs
  
• Call of Duty: Modern Warfare 2  Multiplayer - 11.7 hrs

 
It should be noted that I often play single player games in offline mode on steam, so not all my hours playing games on steam are counted.

I'm just curious, what are the benefits of playing single player in offline mode? "

" @NekuSakuraba said:

" @MattyFTM

• Team Fortress 2 - 127 hrs

• TrackMania United - 100.1 hrs

• Trials 2: Second Edition - 19.2 hrs 
• Saints Row 2 - 13.7 hrs
  
• Call of Duty: Modern Warfare 2  Multiplayer - 11.7 hrs

 
It should be noted that I often play single player games in offline mode on steam, so not all my hours playing games on steam are counted.

I'm just curious, what are the benefits of playing single player in offline mode? "

 There aren't really any. But whenever I haven't got an internet connection, I haven't got Giant Bomb & other websites distracting me from playing games, so I get quite a lot of gaming done when I haven't got an internet connection. That's why a lot of my single player games will be played in offline mode. "

Team Fortress 2 - 1221.4 hrs 
Left 4 Dead 2 - 230.5 hrs 
Battlefield: Bad Company 2 - 142.4 hrs 
Portal - 2.9 hrs 
Recettear: An Item Shop's Tale - 1.4 hrs 
 
Are these hours from Steam accurate? I don't think I actually played that much especially since I only got TF2 last year.

1. The Elder Scrolls III: Morrowind - 56 hours
2. Torchlight - 51 hours
3. Star Wars: Knights Of The Old Republic - 17 hours
4. Altitude - 16 hours
5. Alien Swarm - 15 hours

It says I have 104 hours in Street Fighter 4 because I left the game running over night a few times on Seth because he's a son of a bitch bastard ass fuck whore twat. And 84 in Plants vs Zombies was ran in the background for a couple days while I farmed gold in my spare time.
 
In reality the most active time spent would be in Mount and Blade Warband. However I was playing the non-Steam version up until the recent achievement patch because the massive battles require as much free resources as possible. So the total hours only show 53.    

Dawn of War II @ 89 hours
Civilization 5 @ 62 hours
Borderlands @ 48 hours
 
I also have a couple hundred uncounted hours between Oblivion and Fallout 3 from before I had the Steam versions.
 
And that's all the high ones for what I have installed.

Dawn of War 2 and the expansion - 78+21 hours
Borderlands - 59 hours
X3: Terran Conflict - 53 hours
Medieval II: Total War - 25 hours
Torchlight - 22 hours

Supreme Commander 2 - 29 Hours 
Half Life 2 - 11 Hours 
Audiosurf - 10 Hours  
Team Fortress 2 - 9 Hours
Garrys Mod - 9 Hours 
 
SC2 is an awesome game, I don't care what bitches say. Also, 10 Hours of Audiosurf is a fucking lot.

League of Legends- 1,067 hrs.
 
 
Fuck.

Peggle Nights - 33 hours
FarCry 2 - 32.4 hours
Bookworm Adventures 2 - 28 hours
Yosumin! - 25.7 hours
Aquaria - 22.4 hours
Eufloria - 20 hours
Audiosurf - 18.3 hours 
STALKER: Clear Sky - 18 hours
Batman: Arkham Asylum - 18.1 hours
 
Top 9. Boo-yah.

I only play SP games once and I don't play MP games much at all. The casual stuff I play while listening to podcasts and whatnot, so they get a lot of mileage.

" League of Legends- 1,067 hrs.   Fuck. "

Team Fortress 2 - ~~740 hours, due to idling; at least 200 without.
Dirt 2 - 35 hours 
Day of Defeat/L4D2 - 26 hours 
L4D1 - 11 hours

" Mass Effect - 79.3 hrs   
Torchlight - 55.7 hrs 
Just Cause 2 - 25.5 hrs 
Bioshock - 22.5 hrs
Warhammer 40,000 DOW II - 22.4 hrs        

 
@TheMustacheHero said:

" League of Legends- 1,067 hrs.   Fuck. "

Jesus. You need help. :/  "

Team Fortress 2 - 170.6 hrs
Left 4 Dead 2 - 31.8 hrs
Borderlands - 17.7 hrs
Half Life 2 - 11.8 hrs
Splinter Cell Double Agent - 6.2 hrs

Borderlands - 79 hours 
Street Fighter 4 - 56.8 hours 
Torchlight - 54.1 hours 
Dawn of War 2 - 19.1 hours 
EVE Online: Tyranus  - 18.5 hours 
 
A lot of these are so out of whack, it doesn't have any of my CS:S ,TF2 , or Civ 4 time, ect.

Civ 5, Civ 4, Torchlight, Borderlands, and the Sam and Max games.

1. Borderlands - 109 hours (Still haven't finished any of the DLC yet)
2. CoD: MW2 - 103 hours
3. GTA IV - 35 hours
4. Supreme Commander 2 - 33 hours
5. L4D2 / Civ 5 / Fallout 3 GOTY / TF2 - 20 hours (They all tied)

" League of Legends- 1,067 hrs.   Fuck. "