Of course that will depend on what scares you - for example if you are terrified of spiders, a huge picture of one would be your scariest painting! But there are lots of famous examples of art created especially to frighten or disturb people. My top answers would include Edvard Munch's The Scream and Fuselli's Nightmare, which are both in this very popular list. This list's number one painting is Breughel's The Triumph of Death, which certainly is a scary painting especially if you imagine what it looks like full size.
But I think the pictures that scare us most are often those we saw at an early age or which reminds us of something that frightened us when we were younger. I've always been terrified by William Blake's Ghost of a Flea, for instance, but it would be hard to explain why! It's probably the idea of something so alien walking around and looking, in a way, human.