Who are your favorite villains in Fantasy and SF?

I curate a new Mind Meld at SF Signal, now live today
What are your favorite villains in Fantasy and SF?
I asked a sheaf of authors, from Scott Lynch to Myke Cole:
http://www.sfsignal.com/archives/2012/02/mind-meld-who-are-your-favorite-villains-in-fantasy-and-science-fiction/