I wish I could say Final Fantasy or Call of Duty but they probably come in 2nd and 3rd.
Acording to Xfire I have played 3,538 hours of World of Warcraft. That's roughly 147 days. And that does not count the countless hours I played WoW when I didn't have Xfire on.
I forgot to mention, I don't play WoW anymore. I think I quit last July and I haven't touched it since. The only thing I miss about the game is the friendships I made.