200 Items Or Less: The Thing From Another World (1951)
A few thoughts: The first evil alien invader movie of the 1950s (at least that I'm aware of), and what a great start for a decade filled with them. Notable for its overlapping Howard Hawks-ian dialogue, and for the fact that when the characters in the movie are in a cold room, you can actually see their breath. It's little touches like this that make all the difference, not to mention the fact that this is already one of the most important sci-fi movies of the 1950s. The characters are well written - they mostly behave like believable people, and the way they interact feels very natural - which makes the danger they're in all the more effective. This isn't a simple giant monster eating tons of people movie, as most of the action takes place off-screen, and the "thing" looks mostly human. Definitely different from the 1981 remake, which is possibly even better in its own way, but that doesn't take away from how great this one is.
Who would I recommend it to? Fans of classic science fiction should give this one a try. Well worth a watch (and rewatch, which I did promptly after the first one).