This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / ×××HOLiC
There's a good amount of Nightmare Fuel in xxxHolic, and notable in that, for the most part, it manages to creep the Dickens out of you without anything graphic. The ending to the episode Contract ranks as one of the most horrific moments in the show.
Episode 10, where they tell ghost stories. The three stories told in the anime is different from the three stories in the manga, so we get double up of creepy.
Himawari's story in the manga: A hotel got an uneven number of rooms, where there should be a room, there's just a wall. The people staying at the hotel keep hearing sounds from there, so eventually they break open the wall, and find a room where "Father let me out" is written all over the walls in blood.
Himawari's story in the anime is even worse: A man kills his wife and decides to hide it from his son. But the son never asks why his mom is gone, and carries on as normal. In the end, the dad asks him if he doesn't miss his mother ... "But dad, mom's been next to you the entire time."
Watanuki tells about how he met a man at a crossroad, who carried an umbrella even though it wasn't raining. He passed Watanuki, and Watanuki just kept on walking ... but at the next crossroad, he meets the man again. And at the next crossroad, again. And the man's eyes ... *shudder*
The ... hand in episode 24 of the anime is very very creepy.
Especially when it starts to chase Watanuki ... and in the end, there's lots and lots of them!
The monkey's paw story. The conclusion where the monkey's paw strangles the woman itself...
Scary hands seem to be a motif for xxxHolic. There's a rather terrifying hand Eldritch Abomination in chapters 10 and 11 of Rei.
After reading the arc where the spider enacts revenge on Doumeki for destroying her home made this troper wary about destroying spider webs since.