J.K. Rowling. It is simply a question of penetration. Seventh Harry Potter book sold over 11 million copies in UK and US in first 24 hours. That is just sick. I understand the appeal that those books have and all the very very clever marketing and full blown merchandising can only be admired or watched with growing horror. Rowling is probably the most important fantasy author of all-time and I there isn’t a rising star in sight who could challenge her.
Reason why Rowling is so important:
Generation of readers looking for more fantasy. This leads to more books published, ie. more good books published.
From literary point of view Rowling’s books aren’t groundbreaking, but neither are they boring bricks. British wit carries the books pretty far on itself. I’ve read only the three first books and those were nice pieces of entertainment. Not as entertaining as books in my top ten of fantasy, but good reads.
Before Harry Potter exploded on the scene I would have voted for Terry Brooks to be the most important fantasy author of all time. He was the first herald of commercialization, the one writer who showed the way to tens and hundreds of hacks. He and the likes of David Eddings and Robert Jordan opened the flood gates of fat fantasy. Luckily there have been some authors who’ve injected some thought to the genre.
I should mention Tolkien. After all, his books are source material for countless of books published in the last five decades. Still, his accomplishments pale when compared to the true greats, even if he did write superior material compared to many. Even with the film version of his second most important book he is just some professor who wrote a classic. Even if quite a few literary critics still deny that LotR is a classic.