My mother is a closet horror movie fan, but the thing that freaked me out most when I was a kid was the TV ad for the 1978 movie Magic, based on the novel of the same name by William Goldman. The entire ad was a slow zoom on the ventriliquist dummy's head while he recited this creepy sing-songy rhyme. There was also a violin playing a single high-pitched note for the duration of the ad. I think it would still creep me out today.