Surprised a few "big hitters" haven't been listed...
"Dawn of the Dead" 1978-While "Night of the Living Dead" is a fine film, "Dawn" is better IMO and it did something that "Night" never did and that was kick-start a wave of movies sharing a similar theme, i.e. the recently dead coming back to kill/devour the living. I saw the movie when it was originally released and the level of violence and gore in it set the tone for movie violence and gore for a generation.
"Legend of Boggy Creek"-Another movie that I saw as part of its original run. The movie scared the hell out of me! Has transcending beyond just being a cult-classic. The filming style was revolutionary for the time and set the tone for many movies that came after including...
"The Blair Witch Project"-I know many dislike this movie due to how they (the folks behind the movie) marketed it– which is a point I really do not understand. I think it's a heck of a movie. A great premise and the actors did a fabulous job. It influenced two-decades worth of "found footage" movies (most of which were meh at best but there are some gems like "Cloverfield").