The Terminator, as played by Arnold, was little more than Yul Brynner’s gunslinger without the cowboy suit. Yul is the original, and better, unstoppable killer android in my book. Terminator arguably has a better plot and is a better movie than Westworld, but Arnie has nothing on Yul performance-wise.
I also liked Harrison Ford’s Doctor Norman Spencer in What Lies Beneath.
Gunnery Sergeant Hartman in Full Metal Jacket
Can’t believe I’m the first to name Angela Lansbury as Mrs. Iselin in The Manchurian Candidate. On the outside, she looks perfectly human.
Could a mod fix the thread title? Been bugging me for days.
Christian Bale as Patrick Bateman in American Psycho
Robert Pattrick as the T-1000 in Terminator 2: Judgement Day
Tim Roth as Archibald Cunningham in Rob Roy.
Krank, the mad scientist from The City Of Lost Children. The character itself is more pathetic than evil, but Daniel Emilfork was certainly one of the most evil and creepy looking person on the planet.
I preferred her as the mass murdering Jessica Fletcher in Murder, She Wrote. 
How, indeed, did we all miss her! She was much more sinister than Meryl Streep was in the remake.
Stellan Skarsgaard in almost anything he’s done, but especially Insomnia.
James Mason as Captain Nemo in Disney’s 20,000 Leagues Under the Sea. An excellent portrayal of a thoroughly ambiguous villain (and he manages to overcome the irony of an Englishman playing Nemo).
Also, for pure campy goodness, Hans Conried as Dr. Terwilliker in The 5,000 Fingers of Dr. T.
What’s wrong with it?
Shirley, you jest?
It has a mispelling. I can’t tollerate that.