Fringe/Nightmare Fuel

Fringe redefines this trope every single episode.

The almost unbearable tension from the start of the teaser to the first sign of gross otherworldly mayhem. Nothing's happening yet, but you just know this poor bastard is doomed... any... minute... now.
The first five minutes of almost every episode. And a good portion of the rest. But especially Flight 627.
- A diabetic passenger on an international flight injects himself with what he thinks is his insulin, but is actually a fast-moving disease that first makes him ill, stagger to the lavatory, vomit on a stewardess, and then makes the flesh dissolve off his bones as it swiftly spreads to everyone else on the plane.
Another episode involves a woman who emits a form of radiation when she's stressed that literally superheats the water in organic tissue. As a test, this woman is bound to a hospital bed and a rat is released into the room. She freaks out when it starts crawling on her, and the radiation makes the rat explode. This example is only made slightly less sickening when you see the heroes trying to re-create the radiation to test it and save the woman, they end up blowing up a papaya in a rather sticky spray that was made ironically more amusing when Walter Bishop refers to said papaya as "The friendliest of fruits"....
- Was that the same episode with the woman whose head exploded in the opening (after killing everyone else in the diner, thanks to that radiation spreading ability)? Yeah...
A gas released on a public bus crystallizes, trapping the passengers inside like flies in amber. I, being one who rides the bus regularly, still get the shivers when I remember the crazy of the day for that episode whispering "I know what's going to happen on the bus..."
A boy is surfing the web when a popup Blipvert entrances him. A hand reaches out of the monitor, deforming it, and grasps the boy firmly by the head, boiling his brain inside his skull.
A college professor is lecturing about virology. He starts to choke and sputter, then collapses. His TA attempts to give mouth-to-mouth, but pulls off when something bulges out his neck and starts to climb out of his mouth: A spiney looking slug.
- Ironically, it's a supersized cold. Which immediately makes the thing Nightmare Retardent for people with a biology or medicine background; for reference real viruses look like this. They also don't get much larger than a few microns across. You can drink that water in peace now, maybe...
The random newsagent who gets a two dollar bill and then his orifices seal up. His nose, his mouth, his eyes, and he suffocates. It gets worse when Walter asks if anyone checked his anus and penis to see if those sealed too. It gets even worse when they try and get the guy responsible; one of the agents is exposed to the same chemical, and the main character tries to save him via a tracheotomy. The skin simply eats the trach.
- This isn't the only series to do this, too. This was such a horrifying example that the series Haven had an episode in which the villain erases someone's face.
"Jacksonville" provides some absolutely terrifying Body Horror with a building from Over There colliding and merging with a building Over Here, fusing the two buildings together, along with all its inhabitants. Made that much worse by Astrid's refusal to dissect the body of a man fused with his alternate. When someone who works every day on fringe cases with Walter is upset by something, you know just how extreme it is.
The Russian cosmonaut that became host to some sort of Radiation Vampire from space.
Some other terrifying deaths include being eaten by an evil mole baby, the toxin the disintegrates your bones (creepy doll optional), a were-hedgehog ripping apart a plane full of people, David Robert Jones' exit in season one and four, THE PREGNANCY IN EPISODE TWO, having your pituitary gland removed while you are completely conscious, walking through walls failure, being bludgeoned with a suitcase by someone sleepwalking and thinking you're a demon from hell, the beetles that rip their way out of their human hosts while they're alive, several instances of brains melting, your heart being cut out and only dying hours after, contagious, immediately fatal cancer, rapid aging, acute exhaustion, spontaneous combustion, nanites, at least two instances of heads exploding, BEING DEHYDRATED ALIVE WHILE SCREAMING/PLEADING WITH YOUR KILLER TO RELEASE YOU, being drain of radiation until you were nothing but ash, a zombie being thrown into a turbine, the giant hookworm thingy, your brain melting, jumping out of a window because you've been infected with a thousand year old virius, a hallucinogen that can make your throat slit itself, being chopped up by butterflies, being suffocated with cling wrap...
Detective Olivia in "Brown Betty" getting trapped in a wooden coffin and thrown into the ocean, simultaneously suffering from claustrophobia and slowly drowning. And Walter is telling this story to Olivia's innocent little niece Ella. Surprisingly, Ella seems fine with the subject matter, as long as it makes for a good story.
The Marionette. The ballet dance. Good grief... the ballet dance.
- The victims in that episode. Holy shit.
The end of this video. And be sure to watch ALL the way to the end.
Walter, Walternate, evil!Brandon , September, the ZFT, and the Man in the X T-Shirt.
- Particularly Walternate's grin in 'Bloodline'. Brrr...
Olivia's deadpan statement "...But, I think he's the man who's gonna kill me." Complete with Scare Chord. Watch here.
Getting erased from existence is fairly disturbing in my opinion.
- You think that's bad; suppose you manage to undo it, and then find out that no one you love remembers you. Oh, and you had a son you didn't even know about until after he'd been erased as well.
- Olivia's situation in season 3 is somewhat similar. Imagine being all alone with a single ally in the entire world. Literally. Not to mention the light deprivation torture, experimentation, imprisonment and potential organ removal (while conscious!) with little or no chance of rescue.
Olivia's nightmare (daymare?) in episode two. Especially Broyles.

Broyles: Were you... safe?

Seeing a corpse that looks exactly like you is a pretty frightening idea.
The episode "Bad Dreams." Especially the beginning. Ironic Nursery Rhyme. A woman alone with her baby in a creepy, empty train station. The train is coming. Someone is coming up to her. The fricking Olivia Dunham shoves her onto the tracks with this utterly blank look on her face.
- The other nightmares, which include Olivia guiding a woman's hand to slit her own throat and guiding another woman's hand to stab her husband, are equally terrifying.
You've just been kidnapped. You rescue yourself because you're a Badass. You wake up handcuffed to a hospital bed. Again. With a sex offender sitting by your bed. Who doesn't like you.
Crazy Walter... lovable old man who likes candy. Walter with his brain rejuvenated? He frigging cuts off William Bell's hand and leaves him in suspended in amber. Holy shit.
- Considering the douche was just standing around, chatting causally while Astrid was bleeding in Walter's arms (shot by his men) and turned Olivia into a Apocalypse Maiden... you can see where Walter was coming from. You can also add William Bell to Nightmare Fuel list.
- The reason why Walter had those pieces of brain removed in the amber timeline. William Bell's plot was all his idea and he knew exactly how to pull it off. Damn.

Walter: *smile* And how are things on your side?

The Observers, especially in "Letters of Transit".
- Windmark smiles. After four seasons of September being emotionless, it's just... not... right.
Reanimating Jessica. Eyes should not be doing that.
- Let's not forget her distorted voice.
- All while talking like she's a little girl