Moby-Dick. Don Quixote, Nineteen Eighty-Four, Alice's Adventures In Wonderland, The Call of the Wild, Lord of the Flies, The Painted Bird, The Handmaid's Tale, 2001: A Space Odyssey, On the Beach, Naked Lunch, I Am Legend, The Grapes of Wrath, Last and First Men, Frankenstein,
Try to avoid "Great Books" lists, though. Ask your closest friends who they read. Or if a literary reference comes up that sparks your curiosity, ask where it's from and read that. Did you like a movie? Look it up on IMDB and see if it is based on or borrows from a book (you'd be surprised how many movies are drawn from books, even if they aren't direct adaptations.
Pick one, read it a while. Give it a chance, some start slow. If it doesn't resonate, move on to the next one. If you like a book, try another book by the same author, or from the same genre/era. Follow your curiosity.