These are all authors I've read more than one book by if not everything they've ever written:
Oddly many of the authors of my favorite books do not appear on this list, either because they only wrote one book (Margaret Mitchell, Harper Lee) or because I've only read one of their books, or because I read more than one but only loved one.
My favourite author, by virtue of his book being my absolute favourite novel of all time, is Victor Hugo. I loved his Notre-Dame de Paris. I recently acquired a 1930's edition of his Les Miserables, divided into two books, but I haven't had the time to read it yet.
Marquis de Sade-Les 120 Journées de Sodome, The Magistrate Mocked, Emilie de Tourville, Florville et Courval, Justine.
Gaston Leroux-The Phantom of the Opera
J.M. Barrie-Peter Pan
Michael Jackson-Dancing the Dream
Aleksandr Pushkin-Eugene Onegin
J.K. Rowling-Harry Potter 1-7
Vladimir Nabokov-Lolita, The Enchanter
Honoré de Balzac-Eugenie Grandet
Anton Chekhov-The Sea Gull, Uncle Vanya
Antoine de Saint-Exupery-The Little Prince
Anthony Storr-Solitude: A Return to the Self
Christopher McIntosh-The Swan King: Ludwig II of Bavaria
Interestingly enough, only a handful of books/plays out of all the ones I've ever been required to read for a class have made any impact on me. These are it:
Harper Lee-To Kill A Mockingbird
William Shakespeare-Julius Caesar
Ivan Turgenev-Fathers and Sons
Theodor Fontane-Effi Briest
Nathaniel Hawthorne-The Scarlet Letter
I'm inclined to say pretty much about 90% of the stuff we were assigned to read in school was garbage, but then I look at Stephenie Meyer's Twilight and realize perhaps there are worse things in life.
Sidney Sheldon (the original, not his ghostwriter)
Mary Higgins Clark (particularly her older books)
Erle Stanley Gardner
The list is not in any particular order (except for Dickens--he's always my number 1 for creating Madame Defarge). Right now those are the ones I can think of. :)
If I had to pick just one, I'd probably go for Michael Ende. I haven't read every single one of his books (some of them are hard to find), but I've loved the ones I have read, no exceptions. Also, Chuck Palahniuk is a fave (Survivor is awesome). Anything by Lovecraft and Phillip K. Dick, Oscar Wilde... I like a lot more authors but those are the ones I reach for the most.
Shakespeare - but I'm not bright enough to get the comedies, so just the tragedies
Anthony Bourdain (with the exception of Medium Raw)