I think, contrary to popular and critical consensus, the original Texas Chainsaw Massacre (1974) is not as gory or poor-taste as its reputation would suggest, but rather something brilliantly understated. It does, however suggest an air of putridity, heat and decay that works wonders as a horror genre piece.

I don't necessarily lump the original into the slasher genre, as it predates most of them and doesn't fall into all of the sub-genre's formulaic trappings.