RAGTIME, of course, is a masterwork, but I'd throw THE MARCH in there as well. Also loved HOMER AND LANGLEY and LOON LAKE. I had a strange obsession with LOON LAKE a few summers ago-- read it two times in a row.
THE WATERWORKS rarely got discussed in the same conversations as some of his better known masterworks but it was a strange and utterly fascinating read. R.I.P. to one of the greats.