The vampire Harkon would be the worst Skyrim Emperor, because he wished to make the sun to be blot all over Skyrim so he and his vampire horde could reign over the northern region. More over, Harkon wanted to sacrifice his daughter as half the required elements to block the sun, which is pure insanity. If this creature named Harkon succeeded in his endeavor and became emperor of Skyrim, things would have been chaotic for all peoples the vampires see as prey which the vampirism could have spread beyond too.
The best emperor for Skyrim in my opinion would have been my created high-elf character, why, because I make the magic happen that's why!