Not necessarily a horror motion picture,...but "A Clockwork Orange", despite being a 42 year old movie, remains undiminished in it ability to shock people who see it for the first time.
Couple more good ones:
"Offerings" (Halloween ripoff before Halloween came out
).......not "Burnt Offerings" which is very good also.
"The Amityville Horror" (first one,as well as the 2nd. one with Burt Young.)
Any Hammer film or old Universal horror film is great as well.............elabora
te gothic sets and costumes, as well as great actors