The Day the Earth Stood Still (1951)

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Far from the CGI-heavy, plot-light 2008 version, this 1951 Sci Fi classic is a character-driven critique of cold war paranoia and burgeoning environmentalism.  Unfortunately due to its age it also slightly suffers from the shoot first ask questions later mentality that plagues movies of this era; though certainly not a deal-breaker in such a fine film.