Following Charles' lead once again, here's my list of the top ten fantasy authors:
(1) Homer - Whether he was one guy or more than one is irrelevant--the author known as Homer set a lot of the patterns for fantasy with is amazing blend of history and mythology. It's amazing how many authors have used the Iliad and the Odyssey as models for their stories.
(2) Sir Thomas Malory - Morte d'Arthur is the model for the courtly romance, blending dark fantasy, legend, and history in a tale that resonates through fantasy to this day. Don't let Camelot fool you--the Arthurian legend is much darker than that.
(3) Chretien de Troyes - While Malory did a great deal to codify the Arthurian legend, de Troyes mysticized it with his tale of Perceval, the hero born, and the Fisher King. Every failed quest owes a debt to this wondrous tale--and no one conveys a sense of surreal, dreamlike mystery better than de Troyes does in the sequence involving the meeting with the Fisher King.
(4) William Morris - I agree with Charles that Morris is perhaps the most influential fantasist of the 19th Century--and oddly enough, he's one of the most overlooked. Of course, that's partly due to the fact that he so many other things so very well...
(5) H.P. Lovecraft - Dark fantasy doesn't get any creepier or more haunting than HPL's dream tales. And his pantheon of elder gods, possessed of an ethos incomprehensible to humans, became a fantasy staple.
(6) Clark Ashton Smith - The most poetic prose-smith of the Weird Tales triumvirate, Smith communicates in words what Ditko delineates with illustrations: his work is disquieting and engaging at the same time, and his style is so rich and evocative that it's hard to believe that he's not better known today.
(7) Robert E. Howard - Without him, there probably wouldn't be a heroic fantasy subgenre. Conan is the model for the field, but it's not the only thing he does well; Solomon Kane, Kull, Bran Mak Morn, and Cormac Mac Art are much more than Conan in different clothes, and are fascinating in their own right.
(8) Edgar Rice Burroughs - Sword and planet, heroic adventure, horror, science fantasy--ERB could do it all. His style defined an era, and his fantasy contribution have become part of popular culture.
(9) J.R.R. Tolkien - In spite of the fact that I find his prose tedious and uninviting, I recognize the importance of his contribution to the field. The fantasy quest that Homer created is refined in this sophisticated blend of original ideas, epic plotlines, and Teutonic, Scandinavian, and Celtic legend.
(10) Fritz Lieber - Lieber brought a refinement and wry wit to heroic fantasy that took the genre in a bold new direction.
If I were going for the top eleven, Michael Moorcock would make the list--it was close between him and Lieber, and I still think that Moorcock's eternal champion concept is a wonderful metafictional device. Other authors deserving of mention: Mervyn Peake, T.H. White, C.S. Lewis, C.L. Moore, Mary Stewart, Edgar Allan Poe, Alfred Lord Tennyson, Bram Stoker, Lord Dunsany ("rhymes with rainy")... there are lots of good choices for the next ten on the list.