Graphics-wise, I don't think there's anything quite so profoundly disturbing as a distortion of the human form. Giant bugs and tentacular, Lovecaftian monsters may be scary as all get out, but to see something which is very human, but also very *not* grabs hold of something deep in the psyche. If it's at all possible, through the distortions and mutilations, to see the creature as human, we almost see the creature as ourselves. Zombies, even slow zombies, are scarier than vampires, werewolves and squiggly space monsters. Why? Vamps usually look like people. Werewolves look like, well, wolves. Even Frankenstien's monster was more tragic and sympathetic than frightening. The squiggly space monsters, of course, are scary because they are so inhuman and unrelatable, but it's a different kind of scary.
Then there's things like the Pyramid Heads, which really aren't distorted, but are definitely "humanly inhuman" and scary as all getout. And, I was so glad when my store stopped carrying the Human Centipede DVD because just looking at the blurry silhouette on the cover made me want to curl up in a ball and throw up.