Note bad list, but I'd have to go with...
System Shock 2 - pretty much all of it...
F.E.A.R - some of Alma's appearance's really were the stuff of nightmares...
Silent Hill 2 - again pretty much all of it but Pyramid Head rape scene in particular...
Cradle levels from Thief 3 - not just scary, but set off one of the wandering 'puppets' and fill your pants as they lurch into rapid motion after you...
S.T.A.L.K.E.R - the abandoned (but not really of course) underground labs, second most creepy setting after Cradle...
Ravenholm I have to admit had great atmosphere... but its hard to maintain a sense of true dread when you leave around handy machines to cut zombies in half... still, those zombies with the black headcrabs were really disgusting...
Try to be reasonable... its easier than you think...
