In my heart, Quatermass and the Pit & Planet of the Vampires vie for the title of Greatest Science Fiction Movie ever.

Even if you find you don't agree with my assessment, they're still worth a watch. Planet of the Vampires scared me so bad as a kid I tried to hide behind the theater seat - lighting, sets, horror. All combined for a terrific experience. Also don't be fooled by the titles. There is no "pit" in Quatermass, and no "vampires" in planet. The former does have the origins of Satan however and the latter has alien parasites.

If you've seen them, shout out. If you haven't, what are you waiting for?