Without a doubt the following films have...
Koyaanisqatsi by Goddfrey Reggio / Philip Glass / Ron Fricke
...All 3 are so vital to this reaching so many people via their ideas, music and cinematography. I first watched this at the Scala in Kings Cross in the mid '80s and I was truly blown away. The Scala in KC was such a great cinema; dope smoking, wild graffiti, alcohol and the audience (a major assortment of psychedelic psycho's, squatters, pimps and whore's!). I was to visit the place on a regular basis!
http://www.koyaanisqatsi.org/films/koyaanisqatsi.php
King's Cross Scala:
http://cinema-architecture.blogspot.com/
http://www.bbc.co.uk/blogs/markkermode/2010/09/my_favourite_cinema.html?postid=101364871
Orthodoxcaveman wrote:
"I was lucky enough to frequent the Scala in Kings Cross in the early 90s not long before it was closed down for illicitly screening A Clockwork Orange. It was everything you could want in a cinema. Slightly seedy (hardcore porn like Thundercrack and Cafe Flesh were regularly on the bill), a bit bohemian (chocolate cake) and located in London's decaying red light district. I saw the Exorcist for the first time on the big screen there and was impressed by the atmospheric sound design until I realised it was the rumblings of the Northern Line and not an irate Prince Of Darkness beneath my seat
My gleaming local multiplex is just too damn clean."
Faces of Death
http://www.imdb.com/title/tt0077533/
I saw this in the early '80's and parts of it remain with me today:
"One scene shows a group of tourists in Egypt smashing a monkey's head while still alive and eating its brains"
I can't say it is a great film but seeing it at a fairly young age (13 ish), when all I had been witness to we're mainstream family fare, had an amazing impact. I quickly became vegetarian after seeing this and I never ate any meat again (now 43).
Zombie Flesh Eaters
http://www.imdb.com/title/tt0080057/
This properly scared the **** out of me! I watched the film in a converted old vicarage surrounded by creepy woods. My mate lived in the vicarage and after the film was over (at about midnight iirc), me and my other mate had to wait out in the road for my mum to collect us. All I could hear were zombies coming out of the ground all around us. I had nightmares for weeks afterward and I have never watched the film since! A lot has happened since then, but it has always stuck with me, how a thing as silly as a horror film can cause deep psychiatric trauma in a young developing mind.