I am sure all day I will be remembering other authors and going, "Darn it! I totally forgot him/her! Fantastic author! I'm so silly!" Heh.
1. Terry Pratchett: He saved my sanity during law school, and also writes some of the most "real" fantasy characters I've ever met. A+ for creating characters that are both heroes and flawed in realistic ways. I can read his books a million times and never get tired of them, *and* find jokes I missed the first few times around. Hooray for any author who can make me laugh and stop and think at the same time. P.S. Love to Vimes, Granny Weatherwax, Vetinari, and Death!
2. Ray Bradbury: For The Martian Chronicles, which is like one golden moment of amazement stretched over an entire series of stories; I Sing the Body Electric, (short story) which is a gem; The Illustrated Man, a collection with not a single weak story; Farenheit 451, which makes very good points; Something Wicked This Way Comes, which is spooooky and awesome; Dandelion Wine, which gives us the perfect summer of youth; and for everything else he ever wrote.
3. C.S. Lewis: I'm surprised he hasn't gotten more than one mention thus far in the poll. Long before I was old enough to get that The Chronicles of Narnia was an allegory for anything, I read it over and over again, not even realizing I was getting hooked on fantasy - what a magical world he created, and how vibrantly he wrote his scenes and characters! I don't know that I can pick a favorite, but The Magician's Nephew may be close.
4. Orson Scott Card: Even if he had never written another book, Ender's Game is amazing and perfectly developed. I never get tired of it. But then, of course, he went on and wrote other good books, including Ender's Shadow.
5. Alan Moore: for V for Vendetta. Watchmen is also quite good, but I have to admit I go against the majority and like V better. It's so cohesive and well-written, with such a strong main character and purpose. (Also the drawings are fantastic, even though that's not Alan Moore as much as Dave Gibbons).
Honorable Mentions: Because I can't help myself.
Neil Gaiman, for Neverwhere, Stardust, Mirrormask, American Gods, Dream, and all the rest - I think Neil lives on a different plane than the rest of us, where everything is more spooky (Note: Neil and Alan Moore and Orson Scott Card were kind of in a three-way tie, here...and now I'm kind of wishing I'd put Neil as 4 or 5. Damn my crush on V);
George R.R. Martin for A Song of Ice and Fire - he is the master of complexity;
Anne McCaffrey for all of the Pern books - kudos for making an unbelievable world so believable, and so long-running a series of stories;
Stan Lee, and anyone who ever worked on the X-Men, for creating such a lovable bunch of misfits with a great message;
Madeline L'Engle, for A Wrinkle in Time and that whole wonderful series;
Roald Dahl, for The Witches, Charlie and The Chocolate Factory, George's Marvelous Medicine, and everything else - he's a master of children's fantasy;
Mark Twain, (yes, I know, an outlier) for A Connecticut Yankee in King Arthur's Court, which got me hooked on Arthurian tales at the tender age of 10;
Mary Stewart, for making the legend of King Arthur so real;
Marion Zimmer Bradley, for The Mists of Avalon and The Forest House, which are both brilliant;
J.K. Rowling, for making the world of children's fantasy be both fun and scary at the same time;
J.R.R. Tolkien for sheer world development, as well as idealism and nobleness of purpose in stories;
Kurt Vonnegut, for many things, but particularly for Harrison Bergeron;
Isaac Asimov, for I, Robot and more;
Stephen R. Lawhead, for a fantastic portrayal of Celtic Merlin;
Elizabeth Moon, for The Deed of Paksenarrion, particularly Book 1, which made reading about military strategies and life so much fun; and
Douglas Adams, for sheer ridiculous hilarity.
...And I'm done. Although I'm sure I've forgotten some. Darn it.