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If you mean which writer creates the most plausible/realistic dialogue (where you feel "ah, yes, this is just how people talk") I couldn't say. But for brilliant, witty, urbane dialogue, where people talk as you wish you could talk, then obviously Oscar Wilde. Aldous Huxley is also great, especially in his early novels. The same is true of Evelyn Waugh (read the Anthony Blanche passages in Brideshead Revisited). Of course, all three had been educated at Oxford, at a time when conversation was considered an art. No one speaks like that now – no one can! I'd also suggest P G Wodehouse. The dialogue between Bertie and Jeeves (Bertie, the imbecile, speaking a weird mix of Edwardian upper class slang and '20s Jazz, and Jeeves the urbane butler, replying with a wonderful, polished scorn and contempt).
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When I think of great dialog I think playwrights like Tennessee Williams, Tom Stoppard, Eugene O'Neil, Tony Kushner, Arthur Miller, Edward Albee, James Goldman, Paddy Chayefsky, Oscar Wilde, etc.
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Wilde! The Importance of Being Earnest is an all time favorite.
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I thought Anthony Powell wrote great dialogue, if you're into 1930's Oxbridge talk. George Gissing used to write long, two-handed confrontations. Elizabeth Gaskell used to be great at writing monologues, but I am not sure they many of them could be classified as dialogues.
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I should have added Gaddis to that list.