TOD: Well first, I have to say that "Let's Scare Jessica to Death" was one of those films described as a "lurid melodrama" that I tuned in for when I was young. I didn't find it terribly scary but it was pretty creepy. There was something about those low-budget, lurid films that made them scarier because their low production value gave them a real, almost documentary-like quality.
I like the cheesy horror films that Mystery Science Theatre used to run: The Crawling Hand, The Attack of the Eye Creatures, The Attack of *Anything*, actually would probably be good.
As a child, I liked the anthologies, like "Torture Garden", which usually had 3 or 4 stories. If it was too scary, you knew that it would at least be over sooner than a feature-length story.
"The Creeping Flesh" is a classic, written by a dear friend and editor I used to assist in London. Christopher Lee, Peter Cushing and a mummy---what's not to love?
Since I grew up during the Cold War, there was a proliferation of movies about things that went awry after being contaminated by radiation or something equally as evil. Things that grew big or mutated into something horrific. I went through all the "duck and cover" drills and these films seemed to contain the nugget of a possibility that maybe this could happen, so that made them scary to me.
The two tiny women in the Mothra film still scare me!
But the two scariest films I remember as a kid weren't classic horror films. Besides the flying monkeys from The Wizard of Oz, the two that scared me the most were "The 5,000 Fingers of Dr. T", and "The Day the Earth Stood Still". (Speaking of theremins!) When I saw the second one on TV, I was young enough to really believe this could happen!