Definitally morrowind to oblivion. From a programmer's point of view, its not even a matter of opinion as it is fact.
While i agree that skyrim has amazing graphics, that are waay better the oblivion, its still nothing in comparison from morrowind to oblivion.
Ive played excessive hours of all 3 games, and when i went from morrowind to oblivion when it first came out, it felt like an entire new engine - which it wasnt, but it was that much of an improvement graphically.
Whereas lots of times playing skyrim when it first came out i was always think that it felt like oblivion with better textures and some dynamic shadows (that oblivion originally was going to have, but xbox 360 wasnt good enough to run it the way they had the game programmed at the time, so they had to cut graphics big time to accommodate the 360 because the system itself was still new and not optimized to its full).
But to me, it felt like skyrim just had all the graphical things that were cut from oblivion in the first place. The weather still functions the same as it does in oblivion, the weather effects are just visual things that are centered around the player. If you walk up a mountain a certain distance, you step over an invisible threshold that changes the weather from sunny to snowy. Slowly it will fog up and snow will start coming down, but if you take 2 steps backwards and cross that same invisible line, the weather will turn back sunny again. The weather system itself isnt improved at all except that theres more weather effects (visualizations pretty much) and they tried making the transitions slower. Also, rain and snow still goes through every object like its not there, whereas a game like red dead redemption thats 2 years old has a bigger land mass and its weather doesnt go through solid objects.
Again, im not trying to criticize the graphics of skyrim, i personally love them, but im just trying to show examples as to why its not the best graphic game out there AND how its not that much of an improvement over oblivion.