The Regenerators in Resident Evil 4 were terrifying. They moved slowly and loudly, making gasping, jittery breaths, usually down narrow corridors while you shot the shit out of them, to no effect. What's worse is when you shoot off one of their legs, thinking "Yeah, why don't come and get me now, ************!", just to watch them slither along the ground crazy fast, leap up, and bite your face off. Also that bloodstained, jumping bag in the dumpster creeped me out.
Somebody really needs to make an Eternal Darkness sequel or remake. That game had the best fucking ideas for messing with you. It pretended to mess with your volume settings and erase your data for fuck's sake. How can you have better ideas than that?