I'm gonna have to say Skyrim. I love both Oblivion and Skyrim however there's simple things that I prefer in Skyrim, like being able to pick up objects and not getting told off for it, mwahahaha. But in all serious, both games have their positives and negatives. My brother personally prefers Oblivion over Skyrim. He likes the quests more. Personally I like Skyrim more because there's so much to do, so many fun quests to do after you've finished the main, lots of places to explore. I love how the game has adapted from Oblivion. Oblivion was a great game and base for Skyrim. :)
I'd have to go with Skyrim, however years ago I used to just spend hours in the arena on Oblivion. And Skyrim is just an all time favourite of mine, everything about it is amazing. I chose Skyrim as trying to play Oblivion now is so hard, just the gameplay and everything. But that is due to the age of the game, and the advancement of Skyrim. Still doesn't take away the fact it is a great game, and always will be.
I just recently ordered Morrowind, Oblivion and Skyrim from amazon. The reason for that is because I once downloaded Skyrim and have now been playing it for years and it is my most favorite game of all tiemz. So I thought I'd do the right thing and at least somewhat support the company by buying the games. Haven't gotten around to playing Oblivion or Morrowind yet but I'll get around to it eventually.
But, obviously, Skyrim is da best in my opinion. People have their complaints about Skyrim but I have seen plenty of Oblivion and Morrowind as well.
Oblivion was clearly bigger and the addition of the King of Madness DLC was also pretty awesome. Yet the graphics and voice acting sucked compared to Skyrim which were released on the same generation of systems, so it's a matter of whether you prefer looks to gameplay.