If he gets Mace Windu as his master, he'd become a dangerous duelist. Imagine a Force sensitive, trained in both the Jedi and Sith ways, mastering both Vapaad and Niman. That's scary.
I don't have a specific one, but I have tiers of sorts.
Examples from the top tier of my hated characters would be Nathaniel Essex for what he did to Scott Summers and Jean Grey.
Scott Summers for his constant cheating on his partners, then gaslighting them into blaming themselves.
Jean Grey for ungodly amounts of general bitchiness.
In Marvel? For a long time it was Rogue. But recently I'm more fascinated by Jubilee or Jubilation Lee, I find her character incredible and her visual design charming.