I was having a discussion a few weeks ago with a friend of mine about Robert Wise's The Day the Earth Stood Still (1951). For a long time, The Day the Earth Stood Still was the touchstone for "thinking man's" sci-fi, and it's not hard to see why: it's sober, cerebral, and significantly lacking in bug-eyed monsters. Its plea for peace, too, is a refreshing change of pace for Cold War sci fi. But over the last ten years or so, I've kind of fallen out of love with that movie. The political undertones kind of bother me. Klatuu (Michael Rennie) strikes me as a neo-con alien, who is pursuing an intergalactic version of the Bush Doctrine of pre-emptive warfare. He brings "peace" with an ultimatum and a big goddamn stick. If I squint, I see the same kind of xenophobia in this film as I do in other fifties sci fi film, except for the fact that the movie has cast human beings in the role of monster. I'm also uncomfortable with the veiled religious allegory that casts Klatuu as a messianic figure, but that's a whole different kettle of fish.
I find that I much prefer Jack Arnold's It Came from Outer Space (1953) as a paragon of "thinking" 50s sci fi.