This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Cowboys and Aliens
The moment when Jake finally remembers exactly what went on inside the alien ship. Particularly disturbing is seeing Jake's girlfriend being vivisected by the alien scientist. It is made even worse by the manner of her actual death, which the alien treats with such casual nonchalance that at first you think the beam of light running up the length of her body is some form of passive scan- until she crumbles to ash. Aliens Are Bastardsindeed.
The aliens themselves are full of this, which leads to two extremely effective Jump Scare moments.
The alien scientist, and what he does to Lonergan's lover.
The human captives being hypnotized by the light.
Oh god. LUNG HANDS.
What's worse is how they almost resemble human hands. When they first were revealed, this troper thought that the aliens kept live humans shut up inside of them!
The entire movie. It can give a person a gajillion nightmares.
When the alien Lonergan injured tries to return the favor, the tables turn and he falls right under his own gold machine, the whole pile falling right on top of him. Not only do we see it scalding his exoskeleton, he screeches from the sheer agony, until finally he sinks down... and what's left of him partly resembles the classic look of a human skeleton. It gets out one final breath with a lung hand, then it finally dies.