The 1st time a monster popped out in the Suffering, I jumped about 3 feet in the air and my wife fell off the couch. The little Girl from Fear,and the black spots on the floor that suck you down from Extraction Point.
I don't think it's so much the monster as the eviroment. In the GOOD horror games they set you up, the quiet sounds, the dark, it's quiet, too quiet, you know it's comming, you tense up, your heartrate climbs, and then WHAM, out it pops or scuttles, and suddenly it's in your face! You jump and a ICE COLD shot of adrealine hits your heart, you have to KILL IT AND KILL NOW! And when you do God do you feel ALIVE>