What, in your opinion, is the (mythical) Great American Novel? At least to date. A “classic,” or a current one–either would be fine. Mark Twain? J.D. Salinger? F. Scott Fitzgerald? Stephen King? Laura Ingalls Wilder?
It doesn’t have to be your favorite book, mind you. “Citizen Kane” may be the “best” film, and I concede its merits, but it’s not my favorite. You don’t have to love something to know that it’s good.
This is a hard distinction to make, but for today I would give it to "The Handmaid's Tale" by Margaret Atwood. (Incidentally, the film adapation was decent, even if they did change the ending!) It is a personal story that illustrates the larger world where she lives, and it is a warning of what religious zealotry can lead to if we are not watchful. Of course, I am partial to futuristic dystopian novels.