I've only played Oblivion and Skyrim.
So I'd pick Skyrim just because of how borked the leveling system in Oblivion was. Unless you were careful about how you leveled you were actually punished for leveling. And if you knew what you were doing you could make an ridiculously overpowered level 1 character. Not a smart leveling system.
Side Note: My brother has played pretty much all the TES games and he would probably pick Morrowind.
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