Without a doubt, F-Zero GX. I spent a whole year beating all the staff ghosts, crafting an F-Zero AX machine that worked for me (I actually got to field test it in Funland Arcade in downtown Toronto - it will be missed), and generally mastering the game. To this day, I have never come across another such purely skill-based game that I have had the drive (har har) to master so thoroughly.

COD4.
Can't seem to stop playing it online.

last time I checked, I've played over 8 days on it.

Sonic the hedgehog for genesis.

If you're talking Single Game, then Super Smash Bros Melee. I'd prefer that it be Brawl, but it's just that I've got more on my plate, more single player games worth playing and less time to get in touch with friends than I did when Melee was out.

If you're talking series, then DDR, played often enough to warrant purchasing steel pads. Adding In The Groove to that will boost up the time as well.

COD3 with friends, KH1 by myself.......I don't know how many hours exactly with COD3 (it's alot lol) but KH1 was about 75-80 hours

counter strike or melee

Pokemon Red (I'm scared to think if you count the entire series) and the Smash Bros. Melee. So many hours........ I could've invented something to benefit mankind in those hours......eh, oh well. Bring on LBP!

I'd say that about sums it up for me. I've sunk so many hours into FFXI, but then thats just how MMORPGs are.

Diablo 2, but the Pokemon franchise as a whole has taken about as much time from me.

Damn mario kart series and then Smash Bros series.