I'd have to say Oblivion. Skyrim just bored me far too quickly.
In Oblivion, I found the world be constantly lush and beautiful, and even when I went up north it still held some appeal as a beautiful looking land. Skyrim, while it had its moments, really was just unpleasing aesthetically speaking. Too much of it looked washed out and depressing. I often found myself appreciating Oblivion's lush beauty, but I never had that same feeling in Skyrim.
Oblivion also had more appealing quests. Sure, Skyrim had more, but most of them just felt like a boring, ho-hum trek through the boring world. While Oblivion certainly had its fair share of those, there were plenty of quests around that kept me interested, be it guilds, NPCs in cities, or even random things I found around the world. I actually enjoyed working my way through the guilds in Oblivion, but in Skyrim, I don't think I ever finished a single guild because they just got tiring and I never made a connection to any of them. When I entered a city, I looked forward to the different quests I got. But in Skyrim, they just felt like another one to put on the list. If a random person ran up to me in Oblivion, I wanted to hear what they said and go through with the quest. In Skyrim, I just said, "Well, I'm sure I'll get around to it eventually."
There were other things, but those are the two major ones (and I'm too tired to go through every single one of them). I do prefer the combat of Skyrim, but the combat has never been good enough in The Elder Scrolls games to make one better than the other, unless I find one of them so broken it ruined the whole experience.