Skyrim. Easily Skyrim.
Oblivion is a good game. You might want to try it at some point. But Skyrim has a simpler, more inviting leveling up system. It doesn't have enemies leveling up with the player. It also has some of the more fun skills like the shouts and turning into a werewolf. It also has a way to build your own house, get married, adopt up to two children.
Skyrim is just an all around better package. Not trying to knock Oblivion. Because it really is a good game and I would definitely suggest trying it out at some point. But if it comes down to one or the other, Skyrim's just the easier pick.