It! The Terror from Beyond Space

The first manned expedition to Mars is decimated by an unknown life form... which stows away on the rescue ship.

This film's claim to fame is that it inspired "Alien". And you can sort of see the similarities, or at least the general plot: after landing on an alien world, explorers from Earth unknowingly pick up a deadly creature that hopes to kill them all. Beyond that, it is completely different.

The film has its level of "cheese", with people smoking on a spaceship (which seems like a bad idea, but I guess you could smoke on airplanes so maybe it made sense). The alien is obviously a man with a rubber suit. And the way they walk outside the ship in zero gravity... But I think it still ranks a cut above many (or most) of the science fiction films of its day. Even the design of the ship seems like it took a fair amount of effort.

John Carpenter claims the film was shot in six days. I am not sure how Carpenter knows this, but if it is true we could perhaps give them even more credit: this is a production above and beyond the average quickly-produced garbage.