Easily Morrowind and Skyrim. I have 130 hours clocked in on Skyrim. Havent purchased any of the DLC yet. I remember I had a single 200 hour file on Morrowind on the original Xbox, on top of all the other files I had on Xbox and PC in that game.

The longest Final Fantasy was probably X. I literally had a 100+ hour file on that. I think its the only one that even came close to 100 hours in the series. I remember Xenogears being pretty long as well.