I definitely have to say my favorite villain is a tie between
Queen Nehellenia and Sailor Galaxia
For me, they were two of the few antagonists that had a closely related past and story with the rest of the Sailor Senshi. When you find out who they were and what happened to them, you couldn't help but feel sorry.
And it was because of their pasts and Chaos itself, that they became who they are when we meet them in the story. But the characters themselves feel symbolic too. They were both reduced to only know solitude and destruction. When you're only surrounded by negativity, it takes a toll on you. So they resorted to taking their anger out on others. But because Sailor Moon and her Senshi showed them compassion and kindness, they were able to break free.
Like Sora said in the Kingdom Hearts game -
"The heart may be weak and sometimes it may even give in. But I've learned that deep down, there's a light that never goes out!"
That's all for now! See you next time!