I like a well rounded, realistic character, as I do with my protagonists to be fair. No 2-dimensional mustache-twirling "I'm just evil 'cos I'm evil" unless there's a real good reason behind it or it's hella fun (think Andrew Scott's Moriarty). I don't need them to think they're doing good by being in opposition to the protagonist but I want them to at least be a sufficiently well written character that I could believe that they'd have an opinion on the matter & ideally feel some little bit of conflict over it like a real person would.
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It's hard for me to pinpoint exactly what it is, but some factors include nuance, strong characterization (whether their motives are complex or simple, whether they are capable or bumbling), and a strong dynamic with the other characters in the story.
Those are pretty broad brushstrokes, but when those elements are in play I feel like it genuinely adds something. Conversely, it's notable when they're lacking.
You are missing another element! Antagonists are simply characters who counter or go against the main character. Evil, grey, corrupted,, misunderstood, etc doesn't even have to be a part of them! Cruel main characters are a thing and thus, good moral antagonists exist as well.
As for what makes a good one, I'm not sure how to talk about it without writing too many paragraphs. Context is important. Awareness of medium, setting and the rest of the cast influence what makes certain character traits shine in some spaces and look bad in others.
While real life is full of people who never think through their actions and happily do things to hurt others... I just, I guess I'd rather my fiction be not too realistic. I'd rather read works with antagonists who thought they came to a logical conclusion. Not to say I don't love a bunch of work where the antagonists are "flat"! For some works it's really not the point to do character studies. Sometimes antagonists are simply to represent a bigger idea, a grander concept of life. (I'm saying this yet fully agree to your stance on Ganondorf lol)