It can change day by day, so I will give you two.
This:
A great film that doesn't get its due. Robert Mitchum was never better and it is one of the great shames in cinematic history that Charles Laughton never made another movie. The atmosphere and pacing was unlike any other film I have seen. In its way it is the scariest movie ever made.
My second stars a supporting actor who, by virtue of his looks and voice alone, never got the leading roles that he had in M. Everything about this movie, every shot, and every expression on Lorre's face has stayed with me. It is a nighmare version of the Berlin of the time that indicts society as strongly as the killer. There isn't a lot of dialogue, but Lorre's scene where he speaks in his own defense is among the greatest ever filmed.
I know this was a bit long, but I wanted to give some reasons rather than a list. A film buff can get evangelical about his favorites.