It's been said that every story and every hero is only as interesting and good as the story's bad guy and the hero's nemesis - when I consider that idea the first screen villain that comes to my mind is Ricardo Montalbán's 'Khan' from "Star Trek", both the original series and the 2nd feature film . . . like Frazier with Ali, Khan highlighted Kirk's best heroic features. He was intelligent and haunted, deadly and visually stunning.
Along that same line I think of 'Connor MacLeod's' arch-enemy Clancy Brown's 'The Kurgan' in "The Highlander" . . . again, the story and the hero would be less impressive if the villain were not so impressive.
I also think of Christopher Lee's 'Dracula' in the Hammer films - debonair European host in the day, and ravenous beast at night when the blood-lust is upon him . . . and again, the perfect counterpoint to Peter Cushing's methodical and driven 'Dr. Van Helsing'.
And, I think, you can't present a valid list of screen villains without pointing to;
'Nurse Ratched' - "One Flew Over The Cuckoo's Nest"
'Mrs. Danvers' - "Rebecca"
'Reverend Harry Powell' - "The Night of the Hunter" . . . Robert Mitchum is about as scary as a human villain can get.
'Mrs. Eleanor Shaw Iselin' - "The Manchurian Candidate" . . . Angela Lansbury (from "Murder She Wrote" and Disney films) is as evil and creepy as a screen villain can get.