The Ghost of Sarah Bellows
The story it's based on: "Haunted House"
How scary it actually is: Sarah Bellows might be the twisted architect of every horrific thing that happens in this movie, but in the end, the story takes the easy way out with her, so to speak: Stella is able to "defeat" Sarah by simply promising to tell the vengeful ghost's story (the real story), putting a rest to the legends about her being a child killer. There isn't even the overdone horror twist that by "setting her soul free," she actually just gave her unlimited power (à la The Ring). Sarah seems very eager to shed her anger and disappear for good.
Physically, she's not very intimidating, either — most often she appears only as black shadows creeping along walls and ceilings, until finally taking the form of a near-translucent ghost with wispy long hair and a floor-length dress. Sure, her face flickers into a skull from time to time, but in the grand scheme of things, she's not going to be haunting anyone in real life anytime soon. Sorry, Sarah. (Please don't kill me.)