hmm...its a though one for me since they both had advantages to them. I think I would go with Oblivion on lock picking, a gladiator arena, graphics, and less complicated alchemy. Morrowind on the ability to fly, no limits on everything(aka you could go so fast, and jump so high using glitches that you could jump across a city in a matter of seconds) and it was longer, and was not only a an action-RPG, but also sort of a puzzle game in the way you have to use hints to figure out a majority of the things within the game.
Overall I like both of them equally, and the ones before them I have never had the chance to play.
Morrowind was way better....Oblivion wa only good at graphs nothing else...at morrowind you had a bigger variaty of weapons....in oblivion spears crossbows shurikens darts tower shields medium armor berts were not discovered yet....plus in morrowind i had become a general at the imperial army and i took lots of troops and attack the cities...haha that was fun...great battles....=D while in oblivion the only good thing is that there are better graphs(but you can't judge a game by its graphs) and the fast travel/horse riding system..nothing else.