I think a lot of the scariest ones are very specific to their times. Change unsettles people, and horror stories take advantage of which types of change are going on.
@AlphaCentauri Makes sense. Another of the Vietnam era related movie I like is "Jacob's Ladder" its not really horror but unsettling enough to affect you.
@indiauplko
Night of the Living Dead seems tame now, but it was during the Vietnam War/Civil Rights Movement/early organ transplants. A movie where the government lies to you, the unquestioning press is embedded in a militarized police force, scientists tell people to drag their dead out into the street and burn them, the Black guy is the natural leader, the white family man is a backstabbing coward, and the cops kill the good guy in the end would have been very unsettling to people.