The Day The Earth Stood Still (1951)

Starring Michael Rennie, Patricia Neal, Hugh Marlowe, Sam Jaffe, Billy Gray
Directed by Robert Wise
(actor & director credits courtesy IMDB.com)

A spaceship lands in Washington, D.C. bearing an alien visitor with a message for humanity, but one he insists he must personally deliver to representatives of every nation on Earth.

A powerful sci-fi classic you'd have to think was especially relevant during the atomic scares of the 1950s, but still strikes similar chords today, this picture is certainly among the best work of all those involved from cast to crew.  A terrific screenplay makes its points without hammering the viewer over the head, composer Bernard Herrmann perfectly captures the paranoia at the heart of the film with an eerie and suspenseful music score,  and Rennie makes the alien Klaatu believable in a simple and direct performance.  Few films are as thought-provoking and entertaining at the same time.