Regarding the hidden basement story, the scariest aspect is the strange symbols on the walls. They seem to hold some sort of dark meaning. For the old oak tree story, the appearance of the crying child figure is really spooky. It's an unexpected and eerie sight. In the piano story, the fact that it plays by itself, as if it has a mind of its own, is quite frightening.
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.
For me, the scariest part is when the driver found the claw mark. It just gives this sense of something unknown and potentially dangerous being on the bus.
The sudden appearance of unexplained figures. For example, seeing a ghostly child in the seat when you know there's no one there. It gives you a shock.
The unexpectedness. For example, in a story about a spooky mirror, you might think it's just a normal mirror at first. But then suddenly it shows a face that's not yours, and that sudden shock is a really scary part. It's the element of surprise that makes dumb scary stories effective.
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.
The suddenness of the attack. For example, when you least expect it, a wasp just dives at you. It gives you no time to prepare.
The sense of isolation. When a person is alone and a strange person shows up, like the man in the motel. He was in a strange place all by himself, and the appearance of the stranger made him feel even more alone and vulnerable. There was no immediate help available, which is really terrifying.
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.
In many pikachu scary stories, the sudden change in Pikachu's behavior is terrifying. It goes from being a lovable Pokémon to something aggressive and unpredictable. For instance, when it attacks without any provocation or starts to make inhuman noises. Another really scary aspect is when Pikachu seems to be under the control of some dark force, and it uses its electric powers in a way that is more harmful and destructive than normal.
In the first story about the deer, the sudden appearance of the deer is the scariest part. There's no time to really react properly, and the thought of hitting an animal is frightening. In the rainstorm story, not being able to see clearly because of the broken wipers is terrifying as it puts one at great risk of an accident.