This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Friday the 13th: The Series
There were a lot of disturbing situations in the series, despite its overall cheesiness. One of the freakiest episodes, "The Long Road Home", involved a body-transferring amulet. A Corrupt Hick with a penchant for stuffing animals (and people) is shoved down the stairs by a chair containing the decayed stuffed corpse of his grandfather and is on the verge of death; rather than dying, he transfers his mind into the corpse. Normally, this would restore the corpse to a living, intact condition, but perhaps being rotten and stuffed with sawdust was too much to overcome. When the madman reappears, he hisses eerily, "Why die?", as if even life as a decaying monster is better than death to him. (Unfortunately, captioning revealed that he actually says "Time to die," which is too trite to be really scary any more.)
His abdomen got slashed at one point, and when he runs out of the house his foot gets caught in a trap set in a nearby tree, at which point he's hoisted up by his ankle and the corpse's rotted insides and sawdust start spilling out due to the wind as he screams in agony.
The episode with "Mesmer's Bauble" is quite disturbing. A homely guy uses it to make himself popular and handsome so that he can become the love of a singer he had a crush on. But then he decides loving her is not enough...he has to be her. Cue a scene where he is literally melting into her body. And if that wasn't bad enough, when the bauble is removed all the wishes are canceled causing him to revert from a beautiful woman to a homely man in an equally disturbing way. And this all happens onstage in front of a live audience.
We never found out what the scarecrow did with the heads it collected.
Veda, the Creepy Doll from the pilot episode "The Inheritance", is evil with a capital E-V-I-L, and the girl who ends up with her is just as bad. (Is the doll influencing her? Was she already somewhat bad to begin with? Or is it a little bit of both?)
No list of this series' Nightmare Fuel can be complete without mentioning "My Wife As A Dog". It's bad enough that the cursed collar starts turning the man's "disobedient" wife into a dog. The ending makes it worse because the bad guy WON. While the collar is eventually recovered and taken back to the shop, the original wife is still very much a golden retriever.
The Rapid Aging deaths in "Cup of Time" and "Face of Evil." Also in the latter, the model who gets horribly burned by igniting her aerosol spray.
The Body Horror in "Faith Healer." Both the disease healed when the preacher first uses the glove, and the cancer which Jack's friend wants healed, are horrifically superb makeup jobs. (But then, the episode was directed by David Cronenberg...)
Watching a "Tattoo" come to life and attack you is bad enough, but when it's a scorpion...?
The entire concept of "Brain Drain" is terrifying—not only is it based on the Real Life practice of trepanation that is already Nightmare Fuel, but it goes beyond that into pulling out intelligence and sentience along with spinal fluid. On top of that, the scenes where the machine is used are cringe-inducing between the poor victim writhing and screaming as they are strapped in the chair and the look of ecstasy on the villain's face as he uses it. Having it used on Jack's Old Flame was just the last awful touch, especially when it seems her memories and personality were absorbed into the villain...
If you need any more reason to be afraid of dentists, "The Electrocutioner" gives you one more, where the dentist's chair is actually a cursed electrocution chair that literally dissolves you into energy when you're strapped into it, for the villain to then absorb when he sits in it. The way he received these powers during his botched execution and what happens to him in the end are also rather horrific.
The cathedral radio of "And Now the News" is both an oddly literal version of this and a Meta implication of The Fourth Wall Will Not Protect You...since it works by making the listener hear a cheery announcercommentating on their horrible fate related to their personal fear, one they cannot evade since the radio keeps them from leaving its presence until the death has been carried out. Since this invariably occurs when the victim is tuning in while in bed, it's as if their secret nightmares have been brought to life. And since the viewer is listening too...
Not to mention the ending, where it informs the heroes that it can find the rest of the antiques in no time if "certain requirements are met..."
In "Better Off Dead", it's a toss-up which is most horrifying: the nature of the villain's experiments that result in wild, savage victims drained of a certain chemical in their brains, the fact this happens not only to a friend of Micki's but also (temporarily) to Micki herself, or when the villain's daughter (for whose sake he was carrying out his scheme, to create the vaccine needed to heal her hyper-violence) attacks him and literally tears his throat out.