As far as gore in America goes, The Texas Chainsaw Massacre is a masterpiece. It's also one of the best slashers I've ever seen, which should come as no surprise since Rob Zombie's House of 1000 Corpses, which I love, seems to have been inspired almost entirely by it. It's an intense descent into insanity that manages terror and hilarity through the same means. I can't remember the last time I experienced so many purposefully cringe-inducing moments in a movie, and that's not to mention the effective suspense techniques that put the film a rung above most in its class.
Surely the combination of ruthless kills and the intensity that leads up to them is what gives Massacre its staying power in cinematic history. If anything it is vastly superior to Nispel's 2003 remake and only makes more apparent why that more recent version didn't work. I can only imagine what a theatrical experience would have been like.