Personally, I don't think it's the material, so much as the way you write it. The two books that scared me the most were "The Abyss" and "Christine."
So maybe a story about underwater divers doesn't sound scary - but the author wrote in such a way that I felt panicked and claustrophic the entire time I was reading it. It drew me in, so every shadow in the water made me jump (and want to cry, LOL).
"Christine" is about an evil car. Not scary, right? Oh, but Stephen King is sooo good at forshadowing and making you think "Oh my gosh, something terrible is going to happen".
GOOD horror writing leaves me with a pit in my stomach and a sense of foreboding.
So choose something that scares YOU! Something that you can be soooo enthusiastic about, that your reader is drawn into your world. The subject doesn't matter. It's how you handle the subject.