This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Gremlins
For starters, there's Stripe, the most sociopathic of the Gremlins, especially in the climax when he goes Ax-Crazy trying to kill Billy and Gizmo with a chainsaw. Even as a Mogwai, he had a sinister look about him, not to mention how he gets the dog hung up with Christmas lights. Then there's his Family-Unfriendly Death, when he essentially decomposes at an alarming speed and then pulls a Jump Scare one last time with his skeleton. Even pre-pupa, Stripe constantly wears a sadistic grin on his face.
Gremlin birth. 1) Mogwai -> mogwai. Gizmo-screaming. 2) Pupa -> gremlin. Giant slimy bug-looking things cracking open while Gizmo hides in the motorcycle helmet. 3) Gremlin-> gremlin. Watch the scene with the lightning gremlin and the fire hose. Then try to take a shower without scrubbing your back furiously.
"And that's how I found out there's no Santa." This is the climax of the speech Kate gives explaining why she doesn't believe in Santa and/or hates Christmas: when she was a little girl, her father disappeared without a trace on Christmas night. They never saw him until days later, when the house started to stink. It turned out he had wanted to surprise his little girl by playing Santa Claus, to the extent of trying to come down the chimney for her... instead, he slipped and broke his neck and got wedged in the chimney. And Kate was there when they hauled his decaying body out of the chimney.
The scene where Billy's mom is in Billy's room with the hatched pupas. Silence....then the radio starts playing "Do you hear what I hear?". For Christmas music, it's quite terrifying.
The fact that we still have no idea what happened to many of the people in Kingston Falls; indeed, it's quite possible many of them died.
The Gremlins themselves are pure nightmare fuel. To start, they look like they stepped right out of a nightmare, with mouths packed with needle teeth, long arms with long fingers and long claws, and wrapped in scaly armor. While their aversion to bright light may come across as a Weaksauce Weakness, it also means they gravitate towards dark areas, and they're only about two and a half feet tall. Think about all the times you enter areas that are dimly lit or pitch black, or even just as dim as the areas shown in the films. Now think about all the places in those areas a two and a half foot demon monster could be hiding, ready to spring. . .
And they're not even safe from each other. The Gremlins will off their own kind for cheating, being annoying, or just because it'll be funny.