In the Victor Hugo book, Claude Frollo is a priest. For the film, his profession was changed to judge. I think in the book he wasn't that nasty, either.
I think Frollo is absolutely the scariest because he is so real. There's nothing special (ie. powers, abilities, etc) about him - he's just a man with a twisted psyche.
As for villians I LOVE it would have to be Ursula and Maleificent.... but then again, I'm a Villians addict!! The Villians store is the absolute best in the parks!!