The Day the Earth Stood Still (1951)
Dir. Robert Wise
Amid giant irradiated crabs and attacking UFOs, American post-war science-fiction cinema in the 1950s produced some genuine classics. The Day the Earth Stood Still is one of the very best.
The intelligent script covers a wide spectrum of social concerns, while the contrasts between the military response to an alien visitation and the scientific response (i.e. between men of war and men of reasoning + logic) couldn't be clearer.
Alongside that are more subtle but no less powerful parallels, including an evil among us that looks like us and a determined saviour with a powerful message for the entire world.
Bernard Herrmann's futuristic-sounding score, using electric strings and the wacky theremin, conceptualises the eeriness of another world.