For me, it's Steinbeck. I think his novels capture the uniqueness American spirit, good and bad. Generosity and stubbornness, a contrast of high ideals and actions that don't always live up to them, but sometimes surpass them. They are practical stories about real people we've probably met but don't fully understand.

Cannery Row, To a God Unknown and Tortilla Flat are easy to read examples of this if you don't want to commit to East of Eden and The Grapes of Wrath. Travels with Charley is also a log of America in the 60s, even if you take it as a novel.

I can't argue against Whitman though.