The first is To Kill a Mockingbird. Written by Harper Lee, the novel examines race relations in the American South in the early 20th century with as much nuance and heart as any scholarly work that exists. It also brought the unforgettable characters of Atticus Finch, Jean Louise (Scout) Finch and Boo Radley into American literature. The novel is frequently taught in American high schools, but the story itself is known to those who haven't read it because of the classic film version starring Gregory Peck. Harper Lee herself has not written any major works since To Kill a Mockingbird, and is highly reluctant to grant interviews or make any public appearances. Her reclusive nature might create an era of mystique around the novel, and increasing public interest in the book.
Next is F. Scott Fitzgerald's The Great Gatsby. Gatsby is the quintessential post World War I novel, set just outside of New York City in the wake of the Great War. Though much of nation was suffering economically at this time, the Great Gatsby focuses on a part of the country enjoying extreme wealth, privilege, comfort and extravagance. To make sure that the reader knows that these aspects aren't necessarily being celebrated, Fitzgerald has his narrator, Nick Carraway, be an honest, non-judgmental man from the Midwest, who serves as the moral compass of the novel, though it can also be argued that he might not be the most reliable narrator, and, as a human being himself is also flawed.
Finally, there's Nathaniel Hawthorne's The Scarlet Letter. The Scarlet Letter tells the tale of Hester Prynne, a woman who is unfaithful to her elderly husband, has a child out of wedlock, and is subsequently forced to wear a red (scarlet) letter "A" (for "adulterer") on herself at all times. Like Arthur Miller's The Crucible, The Scarlet Letter warns against the dangers of judgmental nature and mob mentality. Despite Hester's indiscretions, the reader sympathizes with her situation and it is difficult to watch her endure the judgment and scrutiny she receives from the others in her town. The novel's themes of tolerance and understanding make it an excellent candidate for the title of Great American Novel.
It's likely that scholars will never decide what book actually is the Great American Novel, and that's fine. Readers are lucky to have so much wonderful fiction to choose from when trying to make their decision, and with exciting writers being discovered every day on bookshelves around the country, there doesn't seem to be any shortage of contenders. Of course, not every new author is Harper Lee, F. Scott Fitzgerald or Nathaniel Hawthorne, there's new talent to be discovered around the country, and the debate will live on.