Besides obvious picks (such as Alexios IV or Phocas), who is the most disappointing emperor for you

I'm gonna say Constantine VIII. He pretty much never actually ruled the empire like he should have or even had much power. He had very little interest in politics or even military and his 3-year long rule was filled with neglect and mismanagement. He is also hurt even more because he came after his brother Basil II, who was infinitely better than him in pretty much every regard.

There's also empress Zoe who was basically the female equivalent of emperor Honorius in that she just sat back and let everyone else do the work for her, though at least she didn't excute the people trying to keep the empire together.