Karasuman really hit the nail on the head with the remark about Skyrim not feeling revolutionary for its time compared to Morrowind and Oblivion.
I also think it could safely be said that there's been minimal evolution since Oblivion in terms of size and content and gameplay. And, in some cases, a bit of regression
Anyway, I love Oblivion and I dislike Skyrim. I uninstalled it quite some time ago and have no intention of going back - no matter what mods come along for it. The grim and desolate feel of Skyrim is definitely not to my taste. Most of the things which I was very happy with in Oblivion were either removed or butchered in Skyrim.
It's an odd one to bring up first, but the horse riding in Oblivion was fantastic. The only thing Skyrim did better was giving the horse some momentum instead of having it come to a dead stop. But, that aside, Oblivion's riding was infinitely more enjoyable than Skyrim's.
The camera in Skyrim was a definitely step backwards. The lack of proper vanity cam, the off-centre target. Ugh.
Despite Oblivion having a less than perfect UI, at least it WORKED. Skyrim's was more of a UUU: useless, unresponsive, ugly.
The character creation is a weak spot in both games. Without mods, neither is ideal. However, I never thought I'd say this, but I actually think Oblivion's - in all its fat faced glory - is preferable, since it has far more sliders. In both cases though, it's poor at best and the hairstyles in each game are appalling.
While this next one is merely personal bias, I thought Cyrodiil was infinitely more beautiful than Skyrim. And more diverse.
I must say, I can't agree with those who said the graphics were much better in Skyrim. I think SOME are, but most really show the results of a shitty port and the vegetation and grass in particular are exceedingly poor and low quality.
The few areas where Skyrim comes out on top would be armours, flowing water, lock picking, northern lights, dragons, and...er....hmmm....
Overall, Skyrim just feels like all the hype about improvements was a pile of...hmmm...sour Nirnroot.
To each his own, but for me Oblivion absolutely has the most longevity and - even in its Vanilla state - it's my personal favourite in the series.

As it is, there's something between Morrowind, Oblivion and Skyrim to suit everyone. Doesn't matter if we don't agree on the 'best'.
More than anything, these games owe a major portion of their entertainment factor to mods.