I would say Skyrim, as I have over 900 combined hours on both 360 and PC. (I had the 360 version first, then traded it in for a PC copy when I got a computer.) Although, thinking about it, I probably have more time clocked in Elder Scrolls IV: Oblivion. I would say at this point, I'd have 1000+ hours, as I keep going back to it.
Single-player only games or multiplayer-based games?