The episode "Dragon's Domain" from the first season of "Space: 1999" (1975) is widely considered to be one of the best and definitely the most nightmare inducing episodes of the series. It is also widely cited as a possible influence for the film "Alien" (1979). I won't go more into the story behind "Space: 1999" right now because it's complex and rather bizarre.
I do recommend the series, especially the first of the two seasons (holding my tongue about how season two fell apart), but if you only watch one episode I recommend "Dragon's Domain", currently free (at least in the U.S.) on #YouTube Movies and TV.
ProTip: Maybe best not to enter that derelict ship.
