Palpatine.
Extremely honorable mentions that I could watch all day:
Hans Gruber (who nearly ties with Palps for me)
Khhhhhhhhhhhaaaaaaaaaan Noonian Singh
Pennywise
Darth Vader
Boba Fett
Jabba the Hutt
John Doe in Se7en
Christine (the car!)
Jaws (the shark!)
Kathy Bates in Misery
Nurse Radchet (3rd vote!)
Khan Noonian Singh a second time 'cause "Khhhhhhaaaaaaaaannn!"
Clarence Boddicker
Maximillian from 'The Black Hole'
Malcolm McDowell in 'Clockwork Orange' and 'Blue Thunder'.
The Nazis of 'Raiders'
Mola Ramm (Temple of doom)
George W. Bush (in fahrenheit 9/11)
If those guys team up, we, the world are f***ed.