To my knowledge, one of the most significant movies to change the entire outlook on film making in the United States would have to be Easy Rider, directed by Dennis Hopper in 1969. Large studios were losing touch with younger audiences because they were focusing on large-budget flops with big name actors to sell it. Essentially they were looking for the next Gone with the Wind, directed in 1939 by Victor Fleming, but younger generations were uninterested. B-movie Renegade Roger Corman was just handing out cameras about this time to anyone who looked the part of a film maker; one happened to land in Dennis Hopper's hands. Principal photography was shot on-location, and Mardis Gras in New Orleans was later used in the final cut.
The movie was a smash hit, but it also showed those dying studios that cheap art-house movies were becoming a lucrative investment. This single film transformed studio production overnight and revolutionized film making in the states. Location shoots with less than a dozen crewman started popping up everywhere, and anyone with a camera and the knowledge to make a film ultimately had a job. Coppola's 1969 Rain People was a great example. Most of these youngsters were fluent in foreign films ranging from Kurosawa to Godard and were fed up with studio tyranny.
Any film buff can go back and look at the high-water mark of expert filmmaking after the flood gates were opened by Easy Rider. Bogdonavich, Coppola, Lucas, Friedkin, Forman, Milius, Murch, Scorcese, Schrader, and Cassavetes were just a few who were suddenly no longer youngsters but highly-regarded artists.
I am sure that there would have eventually been another movie that could cause the same impact, but Easy Rider was the first and is considered the groundbreaker. The film utilizes the freedom of motion with a camera and the editing is loose and, at times surrealistic by using image flashes and unique framing.
Movie makers today owe the film a debt of gratitude for keeping the United States up to speed with the rest of the world in filmmaking, and who knows when you yourself may need a little influence.