Rasputin: The Mad Monk (1966)

Directed by Don Sharp.

Starring Christopher Lee.

IMDB

One of my favourite Hammer Horror films thanks to a nasty and memorable performance by the classy Christopher Lee. He is just so convincingly evil that he more than makes up for a somewhat lackluster plot and lack of atmosphere.

The only more menacing character I can think of is Spencer Tracy's take on Dr. Jeckyll in the 1941 version of Dr. Jekyll and Mr. Hyde (make sure to check that one out, even if you're not a fan of older horror films.)