Rocketship X-M (1950)
Sure, it's not the most well-made sci-fi movie, but Kurt Neumann directed it pretty well and they did the best with the budget they had (even though that doesn't excuse the horribly lazy make-up on the aliens). The social commentary on sexism too was nice (as she turns out to be way smarter than any of the men berating her), and it's just a fun, enjoyable story to follow. The ending is also applaudingly bleak and depressing, I did not expect that at all.
I think it's really underrated.