According to my steam hours, Skyrim is my most played game. However, I think if I count games I played before Steam even kept track of playing hours, Team Fortress 2 would exceed that.
Before that, it would be a close call between Morrowind or Unreal Tournament 2004.
I didn't make it!
I'd have to say the most time spent on a game for me would be D2. I started playing it when it came out and to this day still play it on occasion. After that would probably have to be WoW.