In 'Stephen Gammell Scary Stories', 'The Dream' is really frightening. It's about a boy who has these recurring nightmares that seem to be seeping into his real life. The blurring of the lines between dreams and reality is a common but very effective horror trope, and Gammell's art makes the nightmares seem all the more vivid and terrifying.
One of the scariest stories might be 'The Red Spot'. It's about a girl who has a strange red spot on her face that keeps growing and has a really gross and creepy outcome. It plays on the fear of the unknown and body horror.