Gamasutra, which used player feedback and biometric data to rate not only the most frightening games, but their most horrifying moments. [Update: The study itself was conducted by Vertical Slice, so if you're looking for dream jobs in the research field, that's where we'd start.)
A victor was picked, but some of the study's other conclusions were more interesting. For instance: The threat of combat was typically scarier than combat itself. Also, gore, while provocative, wasn't, particularly frightening. The game that understood those concepts the best? Well, that would be Dead Space 2, which took the brass ring over Condemned, Alan Wake and Resident Evil 5.
Presumably, Sherlock Holmes: Nemesis was not in the running.