5. Night of the Living Dead (1968)
If this was a list just based around the zombie genre, then Night of the Living Dead would have no competition as #1. George A. Romero made such an impact on horror with his low-budget debut film, that it's almost impossible to watch a horror film that wasn't influenced by it in some way. It's main contribution was definitely in the zombie genre, yet it also set new standards for the amount of on-screen gore and depravity that a filmmaker could get away with. Romero made his film with such cinematic abandon of good taste that every exploitation filmmaker who came to prominence in the '70s treated Night of the Living Dead like the Dead Sea Scrolls. Romero had a child killing and eating her mother, the dead coming back to life to eat the living, copious amounts of gore and, above all, a strong sense of social satire. Romero used the social satire of the film to perfectly capture the essence of the time period in which it was made - that alone propelled Night of the Living Dead into not only one of the most influential horror films, but one of the most influential films of all time.