That is a very tough question. Many ideas come to mind from the obvious like Homer or Plato or the Tao Te Ching and then circled to the usual suspects like Emerson or Thoreau and then sallied to classical but fun writers like Dickens, Hugo, Bronte, and Twain then I think of current authors whose genius will resonate past their deaths and books have reached Italo Calvino, the author of If on a Winter’s Night a Traveller, description of a classic: ‘A classic is a book that has never finished saying what it has to say’. These are authors like Barbara Kingsolver, like Toni Morrison, and A.S. Byatt and Ian McEwan and then took a sharp turn into the books I can never forever put down and also meet Calvino’s description of a classic like C.S. Lewis, JRR Tolkien, James Owen’, Charles de Lint’s Newford books, Stephan Kings Gunslinger series and JK Rowling. The I finally settled on a young readers chapter book called "Wrinkle In Time." by Madeliene L’Engle.
Cause if you can’t pick a "best", pick a "first". This little book has it all including the infamous first line "It was a dark and stormy night…"