I would have to say it's hard choice between two movies. The first being "A Serbian Film" (
https://www.youtube.com/watch?v=lwNLxAKS-Dw) and the second being "Dark Night Of The Scarecrow" (
https://www.youtube.com/watch?v=m9BN-nT16nA).
The first is just beyond fucked up, period. The second was the first horror film I'd ever seen at age 6, and I lived on a farm The kills in that film traumatized me enough to be very leery of being in grain/corn silos for close to 15-20 years, or being outside on a farm after dark.