For me it’s Dracula – Bram Stoker’s original book, unabridged.
Some people far more talented and scary than I confess their fears here at The Washington Post http://www.washingtonpost.com/wp-dyn/content/article/2009/10/27/AR2009102703745.html
Anne Rice: I would say M.R. James’s story “Count Magnus.” That evil could be so easily roused and so relentless in its pursuit of the innocent who stumbled on to it, that terrifies me. But then many of James’s stories are terrifying.
China Miéville: “Sredni Vashtar,” by Saki (H.H. Munro), is a remorseless depiction of stifling, brutal familial cruelty, with a resolution simultaneously satisfying, horrifying, restrainedly implied and overtly macabre. It’s a quite brilliant and terrifying piece of work.