Deliverance

The respectable ancestor of every backwoods-horror film, and a genuine classic. Four Atlanta men take a canoe trip down a river about to be dammed, and the wilderness and its people destroy their weekend and their sense of themselves. John Boorman shoots the Georgia river as beautiful and utterly indifferent.

The assault scene is one of the most infamous in American cinema, and the film's real subject is masculine competence collapsing on contact with something older and harder. Burt Reynolds and Jon Voight are excellent.

Not exploitation, but the source every hicksploitation film draws from.