Morrowind is the answer. Oblivion and Skyrim are just barely RPG's, and their respective game worlds lack the organic culture and atmosphere Morrowind had.

Also, Oblivion and Skyrim may have a trout ton to do, but comparatively little of it is all that interesting.

Daggerfall is a buggy mess. The randomly-generated world just doesn't work.

So, which one is best?