The Day the Earth Stood Still (1951)

Along with a few select other sf films, this one set the template for fifties science fiction cinema. The Thing - the same year - depicted a hostile alien entity from outer space. The Day the Earth Stood Still presents the other side of the coin - the benevolent alien (Michael Rennie). But, such a more civilized version of alien visitors also tends to be more complex; and, it then becomes a question of how much actual benevolence is there for we more primitive human beings?
The posters above are deceptive - they suggest an alien invasion rather than a more benign visitation. The early scenes here are among the most memorable in sf cinema. An alien spaceship is sighted flying over Washington DC and it lands right in President's Park, near the White House. Every moment during this sequence - of the ship landing to the alien's exit from the ship and finally the robot - is marked by iconic, unforgettable images. Gort's first appearance remains a stunner, to this day.
The film really calms down after this first act; the story becomes a mostly quiet exercise of observation of our culture by an intellectual alien. Rennie was a fortunate bit of casting; unknown to American audiences at that point,  he suggests a slightly alien presence  in human form - it's mentioned that his character, Klaatu, is 78 years old (life expectancy is 130 years old for his race, due to advanced medical capability); he does convey this impression. Also excellent is Sam Jaffe as the scientist Klaatu contacts, Patricia Neal as the not-so-average woman Klaatu gets to know and Billy Gray as her son.
Hugh Marlowe as Neal's character's  boyfriend usually gets all the criticism. He's sort of the token bad guy of the piece but I never had much of a problem with him or his character. His limitations were well-suited to the character - he's self-involved, selfish, curt and smug; he's very much the typical modern male who is absorbed with ambition and self-advancement - not hammy evil but definitely very imperfect. He was less effective as a stalwart hero in later films like World Without End and Earth vs. the Flying Saucers (1956).
There are some basic limitations to the story: Klaatu  is visited by a Secretary of State when he's in the hospital, but there's no sign of the President or even the V.P. - this is the single most monumental event in recorded history but is treated as just another milestone, like man first walking in Antarctica or first climbing Mt. Everest. More to the point, only two soldiers are left to guard the immobile Gort and the empty ship - a very blase attitude on the part of the military. This was done so that Klaatu could sneak back later and the boy could spy on the whole scene; these days, I would think there would be tons of curious citizens camped out nearby.

Trivia That Stood Still: footage of the alien saucer flying over the city was used in TV series such as The Time Tunnel and in the episode To Serve Man of The Twilight Zone, which had a similar plot. This was one of the first films shown in the reopened theater in the film The Majestic (2001); the scene seen is the one with the alien visitor first stepping out of his saucer.