Lets see...
Well, number one is Pokemon, without a doubt. I can't quite pin down WHICH one, but between them, I've spent more time on Pokemon than any other series.
A strong second place has to go to World of Warcraft, but only because it's the video game equivilant of crack.
And an honorable mention must go to Bioshock, which I recently replayed for the 7th or 8th time. It's funny, Bioshock is just one of those games where, no matter how many times I play it, it always seems to surprise me. For example, during my latest playthrough, I stumbled upon a small hideaway full of guns that I never found before. I love it when my games do that.