I was talking via phone the other day with an old friend who helped me way back in the early Seventies meet a few of the magazine editors I was trying to sell to.
The subject was what scared us. We both agreed that because of our our age bad health for our loved ones scared us most followed by bad health for ourselves. From there our lists became idiosyncratic.
Number three for me was anything that alters context in a sudden, violent way. You're sitting on a city bus and a guy you suspected of being a junkie jumps up and begins vomiting blood. A sudden assault such as the time when I, in my early twenties, met girl at a pool party who told me to pick her up at seven for a movie. She told me to honk once and she'd hear me. Late summer, long shadows early. I was sitting behind the steering wheel when two large hands pushed through the open window and seized my neck, strangling me. After total panic and terror, I managed to open the door and shove it hard enough to move him back a few feet so I could break his hold. Her just ex-boyfriend of course. She hadn't mentioned him. I took my softball bat (yes I played and badly) with me when I got out of the car. He didn't cower exactly but he did begin apologizing and then he started to cry. He loved her, he was crazy at the moment, he was sorry he'd grabbed me. I'd been there myself so I understood though I still wanted to bend the bat over his head. I didn't of course. I mention this here because four hours later I was still shaking. Virtually my entire body shook. I've never been so shaken before or since.
A close number four is anything that involves tight spaces. I am claustrophobic to a disabling degree. I still have sweaty moments on elevators; can't have medical tests that involve being fed inside a tube-like device without taking tranks heavy enough to knock me out; and I have the occasional nightmare of being buried alive thanks to material I read once for a hsitorical novel--this was on obsessive fear people had in the 1800s. And it was warranted. Premature burial was not that uncommon back then so people asked to be buried with bells in their coffins, strings up top they could tug on, even friends to stand vigil for forty-eight hours to listen for any cries.
As for simple fears, I'd say Robert Bloch still wrote of the spookiest one. Midnight. A frantic knock on the door downstairs. You in your pajamas with your flashlight reluctantly answer the knock--only to find a fully-garbed clown standing on your porch. His crazed eyes apparent. That would sure do a job on me.