For me it's a close call between Dread from The Impossible Dream, Principia Malefex by Ragged Angel, and Kult. Why? Dread reinforces the genre by quantifying story pace and tension mechanically, which I've never seen any other horror RPG actually do (claim to do, yes, actually get done, no). Principia Malefex has the best treatise on tackling taboo subjects (e.g., rape, religion, etc) and 'mundane' evils (e.g., emotional betrayal, embezzlement, etc) in a RPG that I have ever read. Kult is stylistically the most visceral, brutal, RPG I've ever read/played while still managing to retain some substance (it's the Grindhouse of horror RPGs).