Hmmmm... that's a good question. I'm not going to count WoW because it's kind of a cop out and there's no official end. Plus if we're talking about how many 85's I have, it would be 0 with my max at 83 and the runner up at 78.
It can't be Touhou 6, because although I've had several runs of that game with multiple times getting to the final stage (minus the extra one), I feel like there's another game I haven't thought of.
I think I'm going to have to go with the original Ratchet and Clank series for the PS2. They're simple, laid back and easy. But they are becoming very stale recently.
Runner up would be the original Starcraft for the PC/Mac. While I didn't play the single player several times, I played the online stuff like clockwork and it was still kinda fun.