Highest playing time as in actual playing, accidentally left computer on for 24 hours, or just idled for easy money?

Mine most played games

Counter strike global offensive

Pretty much any multiplayer game with a lot of replayability.

For me, it's Dota 2: 1,774 hrs on record

Payday the Heist - 95h

Dark Souls : 133 hours

My most played game is Garry's mod with 427 hours, second place goes to Dark Souls with 397 hours and it might get the first place soon, unless Dark Souls 2 gets released first. Third place goes to Team Fortress 2 with 356 hours. I bought it a long time ago when it wasn't free to play yet, and when it became free to play, even more friends started playing and I played with them a lot.

Civilization 5: 210 hours

"just one more turn..."

1. CS:S - 1,646 hrs
2. CS:GO - 661 hrs( horrible game, dunno why I play it )
3. TF2 - 275 hrs ( another horrible game )
4. The Binding of Isaac - 131 hours ( third horrible game )
5. Portal 2 - 64 hours
6. Foreign Legion: Multi Massacre - 39 hours
7. HL2 - 34 hours
8. Faerie Solitaire - 30 hours
9. Bejelewed 3 - 26 hours
10. CS:CZ - 22 hours

All games that I have played over 20 hours :D

Skyrim 493 hours

Borderlands 2, followed by XCOM:EU.

Football Manager 2013: 500 hrs, but they're like 300 because sometimes I went away and let the game open without playing.

1 Football Manager 2013: 542 hrs (in reality 300, more or less)

2 Football Manager 2014: 92 hrs (not going to play anymore)

3 Rust: 53 hrs

4 Team Fortress 2: 30 hrs

5 Dark Souls: 27 hrs (still playing it)

6 Nether: 22 hrs (5 or 6 of these 22 are for download)

7 Garry's Mod: 15 hrs

8 Hitman Absolution: 12.6 hrs

9 The Walking Dead: 11.1 hrs

10 War Thunder: 10.9 hrs

Dota2 - 585h

Rocksmith 2014 - 60h

warframe 1032 hrs :D

Fallout: New Vegas

Skyrim

Terraria

Sleeping Dogs

Saints Row: The 3rd

Killing Floor

Battlefield Bad Company 2: 751h 08m 27s

over 3000 hours in dota 2 :<

Counter-Strike - 2231 hours.

Garry's Mod with 1,289 hours... I never left it running or anything either, thats all actual playtime. Mostly TTT.
Second is Terraria with 186 hours.

DOTA 2 - 1164 h |
CS:GO - 298 |
Don't Starve - 138 h.

Rocksmith-191 hrs
Dota 2 - 122hrs

Skyrim: 293 hours
Mass Effect 2(my go-to shooter game :P): 166 hours
Mass Effect: 123 hours

PAYDAY 2: 307 hours. Followed by Fallout 3: GOTY 92 hours.