I spit On Your Grave (1978)

An aspiring writer is repeatedly gang-raped, humiliated, and left for dead by four men whom she systematically hunts down to seek revenge.

This film could easily be thought of as just nasty trash, and basically that's what it is. There isn't much humor to it, or likeable characters, or great dialogue, or class. But for what it is, an unrepentant revenge thriller, it's very hard to top its level of cruelty. Obviously movies may have become more gory, and a 2010 remake attempted to top it for that, however I challenge any man to sit through the original's suggested bath tub scene and not cross their legs in terror.