hmmm. I think I would have to go with World of Warcraft.
20 days on my main priest alone. Thats 480 frickin hours!
I have no idea how much time has been buried in Diablo II, Legend of Zelda, Legend of Zelda II, Punch-Out!!, and Super Mario (1-3, Land).
Aprox. 180 hours on Wayne Gretzky 3D Hockey, 150 on NHL Hitz 2002. About 100 Hours on NHL Hitz Pro. 240 hours on Final Fantasy XI (uhg!).
My best Guess would be if you combined Arcade and NES usage of TMNT the arcade game than that would EASILY be 2nd place.
"Let justice be done though the heavens fall." - Jim Garrison
"Ask not your horse, if ye should ride into battle" - myself