Morrowind or Oblivion?  

I prefer Morrowind because there is more to do. There is lots of open space for mods and encounters. The quest chains are much longer. After completing tons of quest, the sky is still the limit. I believe Morrowind has more replay value. Graphics aren't as good at Oblivion. Morrowind's graphics are good enough for me.

I prefer to Morrowind too. I think Morrowind is more complex than Oblivion. And Morrowind ofer more freedom for the player. Ex, you don't need to join mage guild to enchant your weapon. And you can learn levitate. If you like, you can face Dagoth Ur from the beginning of the game, because you don't need to follow the game story.