This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Jonny Quest: The Real Adventures
The nerve gas attack in "Escape to Questworld". You can see the agonizing look on the construction workers' faces as rigor mortis sets in.
The hunter getting mauled by a tiger in the episode "Manhattan Maneater". Seeing nothing but the tiger's claws only adds to the terror.
The greedy developer eaten alive by alligators in "Alligators and Okeechobee Vikings". Though it's offscreen, you can see the movements of the gators' heads as they rend the flesh asunder.
In "The Mummies of Malenque", a mad scientist and self-proclaimed descendant of the Malenque plans to infect the world with an ancient virus. However he gets a taste of his own medicine (well, disease) as his ancestors rise from their graves and give him a good whiff of the virus.
In "The Haunted Sonata", the climax includes the ghost of the original composer of the sonata, who was hidden in the wall of the basement after being pushed down the stairs and breaking her neck, come back to kill the greedy descendant of the man who killed her. She drags him into the wall by the neck and the wall reforms around them, sealing him inside with her red-eyed corpse. Although we later find she didn't actually kill him, we're told by the time he was pulled out of the chamber he'd gone totally insane. We're never told what the ghost did to him.
"The Secret of the Moai" has Race and Dr. Quest de-evolved into ape-like animals in Quest World. That's freaky enough, but when they're removed from the computer, they still are mentally animals and run around, trying to attack their kids.
"In the Darkness of the Moon" features a very unlucky guy who, at the start of the episode, changes into a wolf in a way that sounds very unpleasant. He later saves Jonny from drowning and can talk, but he's clearly in pain and seems to still have something physically off about him, since the camera never shows him directly.
The aquatic monsters accidentally unleashed in "Undersea Urgency" are unrelenting killing machines who claw apart and devour dozens of characters onscreen. When it looks like one has been killed, the head scientist of the undersea base wishes to take the corpse with her so they can learn more about its species... and then the creature awakens in her arms. It immediately cuts back to Dr. Quest and Jonny, who can only cringe in horror as the scientist is ripped apart.
"More Than Zero" was an episode about two paranormal specialists investigating a haunted house in Venice, and Dr. Quest arrives with Race, Jonny, and Hadji to lend a hand. Needless to say, the house is more haunted than any of them would've expected.
The entity in the house pulls the two specialists inside a room where the ceiling begins to descend. The two men desperately try to get out and can be heard screaming their heads off... and then it cuts to outside the door where the ceiling is heard slamming on the floor and the screams stop.
The source of the haunting is a malevolent black pearl containing some sort of entity which feeds off human greed. 200 years prior, the pearl corrupted the original owner of the house and his business partner, empowering the partner and cursing the owner with eternal old age. The man wasn't sure what happened to his partner, until we discover exposure to the pearl turned him into some kind of horrible octopus like monstrosity living hidden inside a sealed room in the house.