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).