For me, the scariest part of the 1999 story was the Columbine shooting. It was so unexpected and the violence was extreme. Innocent students and teachers were just going about their normal day and then suddenly faced this horror.
The scariest thing could be a ghost. You can't see it clearly in the dark, but you can feel its presence. It might pass by you with a cold breeze, or you might hear its eerie moans.
The scariest thing could be seeing unexplained apparitions. Just imagine seeing a figure that looks like a patient who passed away long ago suddenly appear in front of you in a dim hospital corridor. It gives you the chills.
The scariest thing could be the feeling of being watched when you're all alone in a strange hotel room. It gives you this sense of unease and makes you constantly look over your shoulder.
The scariest thing could be the loss of control. As the '69 countdown' proceeds, things start to go wrong. You can't stop it. You might be trapped in a room where the temperature is dropping rapidly, and strange symbols are appearing on the walls. And when it hits zero, you don't know what horror awaits, whether it's being dragged into a dark void or something even worse.
The unknown. You don't know what's on the other side. It could be a monster, a ghost, or a deranged person. Just that small circle of the peephole holds so much mystery and potential danger.
The scariest thing could be a mutated fish with huge teeth and glowing red eyes that attacks the other fish and even tries to break out of the aquarium to reach the humans.