Easily 4000+ hours on COD4 (ps3+pc), followed by maybe Lineage 2, mario kart 64, smash bros and the first lbp game

If you're counting any MMO then I have over 18 thousand hours on World of Warcraft (yes over 2 years) across all my characters. Such a huge chunk of time spent on 1 game. I only spent that much because I was amazing at the game.

My highest on Steam is Counter Strike: Source with about 400 hours.
I have a lot of time on CoD4 on PS3 too. Max prestige and max rank on it.

Can anyone beat my best time yet? 18K on WOW is hard to beat.

i stopped playing for infinity time.
instead, i try to play more games.
but this year: 110h of rayman legends, 110 of DKCTF, 105 of SM3DW, 70 of zelda SS, 65 of dragons crown, 60 of MK8 and 50 of GTAV.

Monster Hunter Tri on Wii.
About 1200 hours.

Probably
Mabinogi - i played it for 4 years (i cant get how long i have played)
And League of Legends

But for consoles, its Lego City - about 400 hours
And on Handheld, its Mario kart 7 - about 300 hours

600+ hours Star Ocean 3: Till the End of Time

Hmmm. I'm going to guess Monster Hunter 3 Ultimate, somewhere in the 300+ hour range. It's likely that an older game like Melee or something is actually much higher, but that was before the activity log, so we'll never know.

Runescape... 186 Days.. 4.4k Hours...