This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Space: 1999
The very premise of the series scared some in a way Star Trek never did. Whenever the base lost personnel, and equipment, that's it. There will be no replacements, and they are in growing danger of running out, especially since they apparently have no production facilities. That makes every death of Red Shirt characters feel like a horrific loss.
Both played straight and partially averted in the Expanded Universe of novelizations and comics, where the Alphans were occasionally shown to have some limited mining and production facilities, but still had chronic supply problems. (Especially in season 2, where a crucial metal called "tiranium"(sic) always seemed to be in short supply.)
It's debatable that they had no production facilities. In the first episode, Alpha was likely involved in the construction of the Meta Probe, and having a construction base outside Earth's gravity well would be one of the best justifications for having a Moon base. Alpha's Technical Section was the largest section on the base, and included rocket propulsion engineers, general maintenance crews, mining personnel.
The episode "Dragon's Domain", and that scene with the tentacled monster dragging screaming personnel into its mouth and spitting out dessicated, char-broiled, smoking corpses...
An episode involving a pair of twins with some kind of voodoo power. The girl produced a very lifelike clay bust of Dr. Russel's head and proceeded to sink her fingers into the middle of the bust's face. The resulting scream of agony escaping from Dr. Russel's own fingers as she pressed them over her own face still can haunt nightmares.
The fate of Commissioner Simmonds (pictured) in "Earthbound". After forcing his way aboard the vessel of some friendly aliens (who are, conveniently, headed for Earth) and who've agreed to take one Alphan passenger along for the ride (since they have a spare hibernation cubicle), Simmonds awakes inside the glass cubicle only minutes into the voyage. Turns out, the aliens' suspended-animation process doesn't work on humans... and the aliens are asleep, and the cubicle can only be opened from the outside...
It does work on humans, but they had to check the staff to see who'd be most compatible with the system - and Simmonds forced his way in before it could be completed. And just to rub it in, guess who the computer deemed to be the one most suited...
They didn't have to check who was most compatible. Dr. Russell runs tests and confirms that the process would work with any human, but Zantor points out that it would have to be calibrated to the chosen individual. Simmonds suffers his fate because he doesn't allow Zantor to carry out this calibration. The computer selection was simply to choose an Alphan at random, but it's insinuated that the reason Simmonds was chosen was that he was the least essential to the continued survival of the base.
From the same episode, the scene where a Moonbase Alpha crew member named Baxter attacks Commander Koenig with a model airplane, shot from Koenig's point of view.
Balor's dark paintings of people in situations of pain and suffering are the stuff of nightmares.
"Alpha Child" plays out rather like an outer-space version of The Midwich Cuckoos, with an alien changeling named Jarak taking the place of young Jackie Crawford, possessing his body and accelerating his growth. The episode subjects us to such lovely imagery as a grown child trapped within the confines of an incubator and said child psychically torturing his own mother.
"Force of Life" involves an Alpha technician named Anton Zoref (played by Ian McShane) who is transformed into a heat-sucking vampire after being exposed to a mysterious alien force. This results in several other characters being instantaneously frozen to death by way of a simple touch. The episode's climax has Zoref incinerated by a laser beam only to be immediately revived as a zombie while attempting to access Alpha's nuclear reactor - the ultimate source of heat on the base. The shot of Zoref's scorched body with its Glowing Eyes of Doom stumbling across the reactor room is one that will haunt you in your dreams.
The shot of the body farm in "Mission of the Darians" with the nude corpses lined up on the table. While nothing too graphic is shown, it is nevertheless pure unadulterated Nightmare Fuel.
The basic concept of the episode warrants discussion here as well. A massive generation ship containing the last remains of a once-grand civilization has devolved into something out of a post-apocalyptic nightmare following a radiation leak, with the surviving members of the underclass being used as Human Resources for the privileged. Word of God states that the whole scenario was inspired at least in part by the Uruguayan Air Force Flight 571 disaster, in which the crash survivors were forced to cannibalize their dead comrades in order to survive long enough to find help.