Rosemary's Baby still scares/freaks me out. It has one of the best horror movie endings.
Dr. Strangelove the image of the cowboy riding the nuke still evokes strong emotions.
Mary Poppins still holds up as good entertainment for my kids today.
I don't believe you can just use a huge brush stroke and say nothing good was made before the 80's. I might even say there were more quality movies in relation to crappy ones made in the 60's and 70's.