In the Amityville Horror, the scariest part was the presence of the demonic figure. Just the thought of seeing a pig - like, demonic entity in your own home is terrifying. The slime on the walls and cold spots were also very creepy, but the idea of an evil being in the house really takes the horror to another level.
The most terrifying part is often the unknown. In these stories, it could be not knowing what's making that strange noise in the haunted house or who the ghostly figure really is.
The feeling of being watched is really terrifying. In spooky stories, characters often sense that there's a presence observing them. It gives a sense of vulnerability. Take the story of the haunted asylum. The patients there were said to feel like there were eyes on them all the time, which added to the horror. Also, the idea of something being out of place, like a door opening by itself or objects moving on their own, is very scary. It goes against the normal order of things we're used to.
In some of the stories, the sudden appearance of the supernatural entities is the scariest part. Like in the haunted house story, when a ghostly figure just pops up in front of you out of nowhere. It gives you an instant shock.
The scariest parts often involve the strange noises Greg hears at night. It gives a sense of mystery and unease.
For the nurse, the movement of the gurney by itself and the sheet on the corpse moving were the scariest. It goes against all logic and normalcy, especially in a morgue which is already a spooky place. It made her feel that there was something supernatural going on.
The invasion of privacy in smart home device hacks is very scary. Knowing that someone could be watching you through your own camera or messing with your home environment without your permission is a huge concern. It makes you feel vulnerable in your own home, which is supposed to be a safe place.
The sudden appearance of something strange. In the story of the face at the window, the babysitter is just going about her job, and then suddenly there's this creepy face. There's no warning, and it's something that seems out of place and threatening, which is really terrifying.
The feeling of helplessness. For example, in a haunted house story, when you're trapped inside and there's no way to escape the terrifying presence. You can't fight it because you don't know what it is. It makes the story really scary as hell.
The suddenness is often the scariest part. Lightning can strike without warning, like in the story of the hiker where it hit the tree near him so suddenly. Another scary aspect is the power it shows. When it causes destruction, like splitting a tree or hitting a chimney and causing sparks. It makes people realize how vulnerable they are.
For me, the scariest is the voices. In exorcism stories, the possessed often speak in strange voices that are not their own, sometimes multiple voices at once. These voices can be guttural, demonic-sounding, and they often say things that are disturbing or full of hatred.