thw elder scrolls series, I've put at least 400 hours into Oblivion and Skyrim each, only about ten hours into Morrowind because I've only just got it. about 5 into Arena because for some reason I can't get dosbox on my new laptop, and I'm yet to play Daggerfall for the reason just mentioned.
[Edited on 12.27.2012 6:42 AM PST]