Best?
On one level I'd say Absalom, Absalom by William Faulkner or Moby Dick by Herman Melville. These repay rereading, being beautifully written and offering the reader layers and layers from which to ... extract? construct? ... meaning. And both are intense. I think it was William Styron I read wondering how Faulkner could keep ratcheting up the intensity of his books without exhausting and finally repulsing the reader.
On a less conscious level, the books I've reread most often include Something Wicked This Way Comes by Ray Bradbury; The Circus of Dr. Lao by Charles Finney; The Adventures [/Memoirs] of Sherlock Holmes & The Hound of the Baskervilles by Sir Arthur Conan Doyle; As I Lay Dying by Faulkner; The Adventures of Huckleberry Finn by Mark Twain; various compilations of Lovecraft's stories; Ficciones by Jorge Luis Borges; The Land of Laughs by Jonathan Carroll. I figure that says these are, for me, among the best books I've read.
A similar list of short stories might overflow a server but would certainly include Hemingway's "Hills Like White Elephants" & "A Clean, Well-Lighted Place"; Frank O'Connor's "My Oedipus Complex" & "Guest of the Nation"; Lovecraft's "The Colour Out of Space" & "The Rats in the Walls"; Shirley Jackson's "The Lottery"; James Thurber's "The Catbird Seat"; Tim O'Brien's "The Things They Carried"; Poe's "The Fall of the House of Usher" & "William Wilson"; and on and on ...
I'm not sure about "worst." I have probably read as bad or worse than the following, but considering my expectations going in these two were the most disappointing.
Slate by Nelson Aldyne (pseudonym for Michael McDowell, I believe) received a few good reviews and was supposed be part of a landmark series bringing gays into the mystery story as main and positive characters. That latter is probably true, but the mystery part I found lame and the character interactions didn't really work so well for me, either. I found the book as a whole rather thin and conventional.
I don't recall the title of the other novel, but it was by Edward D. Hoch who was an entertaining short story writer but the mystery/s.f. novel I read by him was thin, the s.f. element just the furniture of real s.f., and the characters unconvincing. Mind you, this even when I was in high school and very undemanding.
Randy M.