I love this thread and this question. I'm going to argue that it's Francis Ford Coppola. His Filmography in the 1970's I think captures America, the myth and the realities, for both their glamour and horror, better than anyone else before or after.

1972 -- The Godfather -- This encapsulates the American Dream better than anything else. It captures the arc of an immigrant story, of fitting in with America, and what it takes to succeed.

1974 -- The Conversation -- Chronicles where America is going as technology and politics changes and the impacts on our culture, through one man at the center of it.

1974 -- The Godfather Part II -- The story of an America and the image that it's built up, and the bloodshed necessary to both get there, and to cleanse its hands from the past.

1979 -- Apocalypse Now -- The sins of a nation that's ascended, and how the rest of the world must deal with America, and what it does to the psyche of those charged with empire building.

There's a body of work there that captures America in its totality more comprehensively than what anyone else has done. If I had to reduce this down to one choice, then it's simply The Godfather.