No question about it... For me, the scariest movie I ever saw was called "Quetzalcoatl."
I was about 8-9 years old and saw it in a movie theater on a military base in Germany. The story line was about the Aztec god, Quetzalcoatl, who in the movie had the body of a lion, giant wings and tail with beautiful feathers (they had to tell you they were beautiful as the movie was in B&W) and the head of a dragon. This beast was so narcissistic that if anyone kept one of its feathers, it would fly into their window at night, kill them and take back its feather.
An evil Nazi scientist somehow managed to control this creature and kept it locked up in a cave. He would periodically throughout the movie pluck a feather from a wing and leave it to be found by one of the good guy scientists. Then at night he would release Quetzalcoatl who would fly out, kill the good scientist, and then for some reason I still can't figure out, fly back into its cage in the cave.
I refused to pick up any feather I saw on the ground for the next 15 years. Flat out refused.
Stitchawl