Man this is tough... Since I'm more of a psychological type I'd have to go with the smart, genius-like serial killer villains. The first one that comes to mind is Kevin Spacey's John Doe in Se7en. The second I'd have to also say Kevin Spacey's Kaizer Soze from the usual suspect.
He does great villain acting... and those characters are freakin' crazy good.
I'd say for like action type villains I'd go with The Terminator. He's a freaking unstoppable machine!... well at least till the end of the movie, but I'd be an improved version. Either that or I'd be an Alien or Predator!