This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Alice's Adventures in Wonderland
Alice's Adventures in Wonderland and its sequel, Through the Looking-Glass, and What Alice Found There, were chock-full of nightmarish situations and bizarre mental imagery. The fact that the woodcut illustrations portrayed most of the human characters as hideously ugly with grotesquely large heads didn't help much, either. Examples include:
Alice almost drowning in her own tears. Not only is she so traumatized by everything so far that she bursts into tears, but it almost kills her.
The Duchess's Song which includes lyrics of "shaking a baby every time it sneezes"
Imagine living under a legal system which is based entirely on the whims of a bad tempered Jerk Ass who can have a person beheaded for even the slightest annoyance. And she annoys easily. While the King pardons all of the Queen's victims, he always has to do it in secret and we don't know how long it's going to last...
The train scene in "Through The Looking Glass". Every single passenger on the train talks in unison and say the creepiest things like "poor child she should know where she is even if she can't remember her own name".
"If that there King was to wake," added Tweedledum, "you'd go out— bang!—just like a candle!" Existential terror.
'Twas brillig, and the slithy toves did gyre and gimble in the wabe. All mimsy were the borogroves, and the mome raths outgrabe. Spine-chilling.
The Jabberwocky illustration in the original book.Enjoy.
The introduction to the Tweedles. When Alice comes across them, they just stand there, staring at her, so still and quiet that she mistakes them for mannequins. They do this for several minutes as she gets close them enough to read their names on their collars and contemplate the nursery rhyme. Then they suddenly start to talk and move. In real life, that would easily be a Jump Scare. Then there's their appearances,◊ which can seem deep in the Uncanny Valley to some, what with their little boy clothes and enlarged, caricature-like faces.
Humpty-Dumpty tells Alice he thinks she should have stopped ageing when she reached seven. She objects that one cannot prevent oneself from ageing, and he replies that she could have stopped ageing at seven if only she'd had "properassistance"...
The March Hare in the 1999 Hallmark version. According to Wikipedia, his costume even scared the actress who played Alice.
Martin Short's Hatter in that version is way deep in the Uncanny Valley.
A lot of elements in that adaptation are quite scary: from the realistic puppets of the creatures, to the dated CGI which stands out amidst the practical effects, and such creepy makeup like the Cat's human face amidst a puppet cat's body, or the Queen's pencil makeup...It's scary without trying to!