I haven't seen a lot of the old Doctor Who, but from what I've seen of it and the new stuff, I think right now it's between Omega, Davros, and the Daleks.
Omega cause he looks cool despite how whiney he seemed in the Three Doctors, Davros cause he's creepy, insane and oh so ugly, and the Daleks cause they're quite scary for such silly looking things.
I've only seen the new Master and one appearance of him in a Classic story so I can't judge him overall. I like the new guy though.