Seymour is both my favorite and least favorite at the same time.

Favorite because of his depth. He wasn't a 2D baddie who just wanted to destroy the world for hell of it, or become God to rule the universe. Unlike most of the villains who knew well that they were committing acts of evil, Seymour actually believed what he was doing was right. He had a savior complex. "I'm going to destroy Spira so that no one will ever have to suffer again."

Least favorite because he was a pedophile. And most of all the Seymour Flux form!