Paul, we could pull Webb into the 25-year window with The Emperor's General, certainly his most philosophical exercise.
Of course, the whole idea of a "best" in novels is ridiculous. So many fabulous novels have simply disappeared: The aforementioned Edwin Mullhouse, Paul Rogers' A Child of the Century, Frederick Boechner's four novels about Leo Bebb, Real Money by Louis P. Jones, The Dolly, Dolly Spy by Adam Diment, any of which I'd gladly read again, right now.
If you ever want a depressing experience, look over the publisher's lists for the upcoming year. Book publishing has always been PC, but it's now totally rotten.