There's a lot of possible answers for this one, as you'll see when you take a look at the responses many people have given. But for me, there was one villain who stood out above the rest, a mutant from Isaac Asimov's Foundation series, one of my first encounters with the concept of mind control...
Go check it out:
Mind Meld: Who Are Your Favorite Villains In Fantasy And Science Fiction?
Note that my contribution includes something of a spoiler for the Foundation series, in case you haven't read it yet. But it's kind of an obvious spoiler.