The scariest episode of "Space: 1999" (among a number that were definitely upsetting) is called "Dragon's Domain". There is essentially 100% agreement on this on every list (including mine), and the number of nightmares it has produced over the decades since its original airing in 1975 is undoubtedly immense. It's said that when BBC reran the episode 20 years later (!) they got a pile of complaints about running it too early in the evening.
Oh, you want to see it? #YouTube to the rescue (at least in the U.S.):
