2001: A Space Odyssey
Honestly, I can't think of any major-impact movies, so I chose one that had an impact that effectively promoted two things that I love: space and scifi. 2001 achieved these by being a major, eye-catching, oscar-winning movie that moms-and-pops-six-pack could sit down right next to and enjoy with phds. I think that without 2001, there may not have been a Star Wars. And who knows, maybe there would not have been a dedicated NASA Mars exploration program?
Bonus: the film also promoted classical music.