Agatha Christie (sorry she's not fantasy). She has an extensive collection of books, and all of the ones I have read have been delightful. I'd like to ask her how she managed to have such output.
This is a really tough one. Either Tolkein or J.K. Rowling. They're both great. I'd have different questions for both. Though I might go with Tolkein. Mostly about the creation of the cultures of the various races in his legendarium.
I would like to meet the great Gabriel Garcia Marquez. Who could ever forget the unforgettable novels he wrote like: One Hundred Years of Solitude and Love in the Time of Cholera. Too bad he passed away but his works will be eternal.
Just because he seems like he would be a fun guy to converse with and have interesting things to say, and a lot to show off as far as first drafts and ideas for books.
The man was actually a perfectionist and rewrote over and over. When you read his work, you probably think the words just flowed out naturally onto the paper when he wrote it.
Roger Zelazny. I'd like to discuss writing and some of his novels, of course. And what he would've written/had planned if cancer wouldn't have taken him.
There are lots of modern authors I'd like to pick their brains about writing with, but I guess they have that enough and I'd get preformed answers to my [boring and repetitive -to them] questions...
So I'd go for Lewis Carroll or Edgar Allan Poe. Poe would have to be fairly sober and not in one of his moods, so we could get drunk and rage together, I think a pub crawl with him would be fun... Carroll, I think would be interesting to talk to just to find out what his mind was like being a Fantasy/Nonsense writer and a mathematician [yes - I'd love to discover the "truth" behind Alice...]. And you've got to love the name "Charles Lutwidge Dodgson"...