This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Boards of Canada
The entirety of Geogaddi is deeply unsettling, with deceptively sunny keyboads filtered through tape recorders that make them sound broken and distorted. Particularly creepy tracks include:
Right out of the box with "Ready Lets Go", which is little more than an oscillating drone.
"Music is Math," the second track and the first full song on the album, sets the tone. The lyrics are hard to make out (as they're being sung in a near-incomprehensible, tribal wail filtered through a vocoder) but consensus seems to be that they are: "we all fall down... we all fall down... down..."
"You Could Feel The Sky."
"1969" crosses Boards of Canada's Right to Children-era sunny sound with unsettling vocoder bits and references to cults. In fact, it's about the Branch Davidian cult, which maintained a Stepford Smiler facade while their leader sexually abused children.
"A is to B as B is to C" , with its strange, garbled loops and samples. Towards the end of the track, an ominous backmasked chorus chants something that sounds astonishingly like "O Geogaddi! O Geogaddi!"
"The Devil is in the Details," which is about hypnosis. Not a good idea to listen to in a dark room.
It doesn't help that the backing beat sounds vaguely like an insect moving its wings back and forth.
Speaking of Hell Interface, the Anti-Christmas Song "Soylent Night" is arguably one of the most unsettling tracks ever heard from BOC. The main melody is a sample from the fifth movement of Johann Sebastian Bach's Magnificat twisted into a dissonant, haunting loop. Partway through a passage from "Silent Night" can be heard, but it has been treated in such a way that it sounds more like a faint and ghostly wail than any human vocal. To top it all off, the song concludes with a distorted robotic voice reciting the Lord's Prayer.
"Slow This Bird Down" is fairly ominous, especially compared to the rest of The Campfire Headphase.
"House of Abin'adab" from Old Tunes Vol. 1 practically lives off of Nothing Is Scarier; the only sound throughout the entire track is a low, quavering drone that occasionally pipesup in intensity.