This is a hard question, because there are so many movies that have shaped our culture. If I had to pick two movies, I would pick, Wall Street (1987) and Rocky (1976).
The latter tells the story Rocky Balboa, an unlikely boxer, who beats all odds to become the heavy weight champion. The story is inspirational, because it show the triumph of the underdog. More importantly, it shows the importance of honesty, hard work and determination. These three characteristics were very much part of the American culture and some would say these values made American great. In light of this, Rocky is an all American story.
The former is about Gordon Gecko and his take no hostage approach to finance. His goal is to get rich and he is infamous for saying "greed is good." Ironically, Oliver Stone, the director, wanted to make Gordon Gecko an incredibly distasteful character, but people actually resonated with him. This lead to more and more people wanting to be like him. From this perspective, this movie abetted the financial crash of 2008, as greed motivated many bankers and Americans to make foolish and unethical decisions.
The two films are polar opposites in terms of meaning. However, both shaped our culture in profound ways.