As a kid, (and as an adult), I lived for horror films, and would always beg my parents to let me watch the late night horror show on Friday nights. In Uruguay, where I grew up, these were usually advertised with the equivalent of a G rating, so I usually was able to watch even the most gruesome selections.
Out of all of these movies, there was one that truly frightened me, and haunted me for the next fifteen years, when I was finally able to identify the movie, watch it, and be haunted all over. The movie was INFERNO by DARIO ARGENTO. Two scenes in particular stayed branded in my mind, so much so that when I watched the movie again the scenes were exactly as I remembered and dreamed about them. Towards the beginning of the movie a young woman crawls into a hole in a basement; what is down there are two underwater rooms, fully furnished, almost looking like someone is still living there (why is there an apartment underground and full of water?). As she's swimming down there, she bumps into a rotting corpse, and as she's trying to escape gets all tangled up into it. Unexpected and terrifying, it was unlike anything I had ever seen.
Later, the young woman is attacked next to a lake by a swarm of rats. As she screams for help, a food stand vendor hears her, picks up a meat cleaver, and runs towards her, presumably to rescue her. When he gets to her, though, he holds her down and hacks her up with the cleaver! As a kid, this terrified me: in other movies I knew who the killer was, why he was killing his victims, and usually how he would do it. This movie was so random, nothing was ever explained, that it really unsettled me. Now, twenty years later, I understand the strange logic of the film, but those scenes still creep me out to no end. In short, it's one of my all time favorite movies, and I watch it as often as I can.