I don't expect to find much agreement on this one, but I have asked myself the question before if Robert Penn Warren's novel All the King's Men might be the greatest novel ever written. Warren is not the greatest novelist in history when we look at his body of work, but he certainly is a master of the medium and he wrote this one masterpiece.
I admire the depth and variety of many of the suggestions above and I know we won't come to a consensus pick, so I'll just make my argument for this book as one of many book-loving arguments in the debate.
Out of all the novels I've read, I find the form to be most fully expressed and fully explored with poetry of language, resonance of symbol and theme, and philosophical depth inAll the King's Men.
It's really well balanced, completely tied together, and structured in a way to enhance the significance and the narrative momentum of each section of the text making the book gripping, readable and satisfying.
I believe the best writers of novels worked in the 1920s - 1940s and that Dostoyevsky and Tolstoy are right up there too. But for a single, stand-out A-1, top drawer novel, I'm going to go with the underdog, All the King's Men.