I like George Lucas. But I'd prefer J. K. Rowling if he's not in the library.

Douglas Preston & Lincoln Child, Robert Jordan, George R.R. Martin, R.A. Salvatore, Elaine Cunningham, Carrie Vaughn, J.R.R. Tolkien, J.K. Rowling, and Neil Gaiman are among my top favorites.

... If I had to choose it would be a very very close tie between Preston & Child (together) and Martin. I love that both of their works, while containing many impossible things, have a very solid grounding in reality. Preston & Child's books are set in the real world but have many sci-fi elements to them. Martin's work is fantasy but with a very hardcore grounding in the history of the Middle Ages. I cannot choose between the two.

I'm studying how to successfully craft well-written SF/F novels, and my teacher is a very popular SF/F author who has published 15 books including the well-known "Star Rigger" series. His name is Jeffrey A. Carver, but sometimes his books are published under the name "Jeff Carver".

I would have to say that my favorite author is Orson Scott Card. He is able to blend action with strategy with politics with adventure with AWESOME in just the right way! Most of the books I've read of his don't feel too heavy with one element, nor too light with another. He's great!

This is a very good question for me.  I have so many authors that I love that it's hard to narrow them down.  Tolkien deserves a mention for getting me interested in this genre in the first place, but my favorite?  I don't know.  I think I'm going to have to split it up between Sci-fi and fantasy.  On the sci-fi side I'll have to go with Robert Heinlein.  He had such a huge body of work and I have enjoyed every one I pick up, with 'Time Enough for Love' being my favorite (I can't get enough Lazarus!).  His stories manage to cover such a range of the human experience that I often finish a book and end up thinking it over for the next day or so.  Now for the fantasy side.  This was a really hard pick for me, but I'm going to go with Terry Brooks.  Over the last 30 years he has artfully developed the world of the Shannara series, tying in more than 20 books to create a world whose history now spans several thousand years.  But like I said, hard pick.  A quick list of honorable mentions:  Robert Jordan, Stephen R. Donaldson, Orson Scott Card, George RR Martin, Raymond Feist, David Eddings, and RA Salvatore.

I would say C.S Lewis

Steven Erikson. Easily. The Malazan Book of the Fallen was an amazing series. But also right now Jim Butcher, Joe Abercrombie, George RR Martin. But Erikson is the best. Read it.

I have to say JK Rowling. I adore the Harry Potter series and always will. [= She created such an amazing world, and her books have actually made me cry. Not many books have made me cry. I actually cried when the series ended. 

Anyway, I also love Jim Butcher, Douglas Adams, CS Lewis, and JRR Tolkien. Douglas Adams, especially. The Hitchhiker's Guide to the Galaxy is one the funniest books I've ever read (and it's miles ahead of the movie).