starcraft for sure... probably 300 hours including multiplayer

There are three games that comes into my mind...Football Manager,Civilization 3 and Morrowind GOTY.

Starcraft, counterstrike and Warcraft 3 (DOTA)

WoW easily... but in more "fair" terms probably FFVI and FFIX as I've played through both 3 times and put at least 80 hours into each playthrough. After that Pokemon Diamond and Pearl. 2 save files (Diamond and Pearl) one with 180 hours another with 30 and counting. After that probably Harvest Moon: Friends of Mineral Town on GBA.  I'd estimate I spent at least 150 hours on that game. 

WoW is up there on my list even though I don't like to think of that game period. Probably totaled above 2000 hours. CS: Source and COD 1 are above WoW though. Above those is Halo anything.

Probably SSB Melee. Dunno how much, but it must've been over 1000 hours. :p

Kingdom Hearts II.

Played for about 57 weeks, 14 hours each week. That means a total of 800 hours.

Super Smash Bros Melee by a mile. But that might change if I continue playing SSBB.

It would have to be Gears of War, I still play multiplayer despite having other games such GTA 4 and Halo 3.

Dark age of Camelot and Age of Empires II online, thousands of hours.

Oblivion is my most played single player game in recent years, a bit more than 1000 hours.