What Makes For A Good Antagonist?

For stories with an antagonist, what do you like to see? What antagonists, canon or fanfic, do you enjoy?

For example, I tend to like the fanon interpretations of Ganondorf more than how he is in canon. Considering how canon doesn't tend to dive into his character too much beyond "power". Whether fans write Ganondorf as an antagonist in reform or still straight up "evil", they still add in nuance about what it is like to be one of the rare male Gerudo or some awareness of the eternal cycle of the Triforce.

On the other end of the spectrum is Alex, from the original Golden Sun duology. Something about not knowing him very well stuck out to me. Despite his motivations being very simple, how he interacted with the protagonists of the game was interesting. You never faced him directly. He was an instigator for many things and probably wanted to be the most important person in the story. But he wasn't.