Well most of the good ones are guys so guess I'll just pick my favorites. I loved Bill the Butcher in Gangs of New York. Also Mr. Blonde in Reservoir Dogs was great! Loved Castor Troy in Face/Off and Tim Roth in Rob Roy. But of COURSE there's Darth Vader!! But I think the scariest villain of all was that creepy ass girl in The Ring!!
NICE, while i was reading your post I was thinking of some of the crazy chicks in the asian horror genre. The creepy girl in the movie MAY was something else as well, hahaha. If you're thinking horror movies you will come up with all kindsa evil chicks.