Don't know for sure, in order is something like Mario Kart 64, Super Smash Bros Melee, Unreal Tournament and Gran Turismo 3. Tought the only one I've got almost tracked is Melee, I've played about 500h on my console, not counting when I played it from friends, so I guess something like 700h.

WINTERMAUL!!! I used to love that game so friggin much!!!!!!!!  But I stupidly sold my copy of Warcraft 3 and still miss it to this day :(

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On topic: It almost has to be Warcraft 3...I used to play that game so friggin much.  If not that, it could potentially be League of Legends, which I've probably put over 200 hours into.  I suppose I put a lot of time into Starcraft 2 for the first month after it was out as well, but probably not near that much.

Civilization II

About 80 hours on Fallout 3.

Guitar Hero III (700 hours)

Rock Band 2 (500 hours)

Fallout 3 (250 hours)

Pokemon Diamond (200 hours)

Pokemon SoulSilver (130 hours)

StarCraft Brood War (186 hours logged while on Xfire)

StarCraft II (32 hours logged while on Xfire)

Halo/Custom Edition (300 hours logged while on Xfire)

Monster hunter tri with over 100 hours and I still want to continue playing but I currently have some other games to finish. Starcraft would be another one and guitar hero. Oh and I have beaten chrono trigger and final fantasy 6 at least 5 times each.

Killzone 2 mp this gen. I collect rpg's so I shudder to think the amount of time I have played my rpg collection especially on my ps2. 

probably Starcraft 2, I'm playing it with my friends several times a week

- maybe I still clocked more play time with dota or counter strike, considering I've played both for several years, but SC2 should overtake those soon

Probably one of these five games:

Super Smash Bros Melee: I've calculated about 600 Hours on my GC, 300 on friends and probably another 100 on Misc systems.  About 1000 hours.

Counter-Strike: Unknown amount of hours between my system, friends systems and LAN centers.  I'm estimating about 1000 hours.

Sim City 3000 Unlimited: I've spent hundreds of hours on all the Sim City games.  But this one probably beats them all.  I'll never know how much but its calculated in months, not hours.

Ragnarok Online: I can't even begin to imagine how much of my life I wasted on thisd game.  All I can say is, it was practically all my friend and I played from 2001-2003.

Pokemon Emerald: My game file says 450 hours.  But it hasn't seemed to add any new hours for a long time.  I'm estimating probably 600 or more hours.

Warcraft 3 and World Of Warcraft for me. Maybe Plants Vs. Zombies too.