1. Faulkner. He came close to winning. But which novel? Absalom, Absalom is the deepest and richest. But you need to read it at least twice in a row, and that makes it less of a story. Here is the first page. As I Lay Dying is the most enjoyable. Read it through once, without trying to understand it. Then read it through voice-by-voice. Then read it through again. Sound and the Fury and Light in August (Faulkner’s easiest major work) cannot be dismissed either.
2. Henry James – The Golden Bowl. Are you interested in Girardian doubles, the triangulation of desire, self-deception, the use of gifts to imprison, the mediation of desire through objects, and the dynamics of marriages? This was James’s last and best novel. For my taste Portrait of a Lady is static and stands too close to the Merchant Ivory tradition. Interestingly, I believe not one of you mentioned James in the comments thread.
3. Huckleberry Finn. It seems more Shakespearian each time I read it. Right now Yana is reading it and loving it.
A few comments: Fitzgerald is not quite there. I am tempted to count Walt Whitman’s Leaves of Grass as a novel, not a poem. Willa Cather’s My Antonia and Nabokov’s Pale Fire are close, although my wife will not let me treat the latter as an American novel. Philip Roth has many excellent novels but no one for me stands out. Only the first third of Gravity’s Rainbow is wonderful. I prefer Hemingway’s short fiction and most of all his sociological non-fiction on bullfighting. Bellow is excellent but I wonder how much his books will mean to people one hundred years from now. The dark horses you already have heard about.