J.R.R. Tolkien - The Lord of the Rings (single work, not a series)/ Silmarillon/ The Hobbit
CS Lewis - The Chronicles of Narnia Series / The Screwtape Letters
Robert A. Heinlein - Glory Road / Waldo / Magic, Inc. / Farnham's Freehold / Beyond this Horizon / The Unpleasant Profession of Jonathon Hoag / All You Zombies, etc.
Jean M. Auel - Clan of the Cave Bear Series Bear is singular
Isaac Asimov - Foundation/ Foundation and Empire/ Second Foundation.
Piers Anthony - Magic of Xanth Series / Incarnations of Immortality /Blue Adept Series
Christopher Stasheff - King Kobold / Wizard in Spite of Himeslf / Her Majesty's Wizard (ALL of these are part of the "Wizard in Spite of Himself" series)
A.E. Van Vogt - The Weapon Shops of Isher series / The World of Null A series / Slan
Anne McCaffrey - Dragonriders of Pern series
Frank Herberts - Dune series
Barry Sadler - Casca: The Eternal Mercenary series
Jules Verne - 20,000 Leagues Under the Sea / Around the World in 80 Days
Dante Alighieri - The Divine Comedy: Purgatory, The Inferno, Paradise
Ursula K. LeGuin - Earthsea Series / The Lathe of Heaven
Ray Bradbury - Fahrenheit 451 / The Martian Chronicles / Something Wicked This Way Comes (The only book that REALLY scared me... I read it when I was 13.
L. Frank Baum - The Wizard of Oz series
Robert Lynn Asprin - Another Fine Myth series
Theodore H White - The Once and Future King
William Shakespeare - A Mid Summers' Night's Dream
I'm certainly glad to see Richard S. Tuttle included in this list! He is my all-time favorite fantasy author. I've read all of his books at least once and am constantly amazed at his ingenious plots and characters. He's right up there with the best of them!