What I've done is also take into account influence on the fantasy genre and storytelling skill. Thus this is not a list of who is the most popular. Its a list of the greats when it comes to the craft.
1. Robert E. HowardHoward was the creator of Conan the Barbarian and by this also the Sword & Sorcery genre. Because of this, he has had enormous effect on the fantasy genre as many authors and directors were influenced by the character Conan and his story. Sadly, Robert E. Howard committed suicide aged only 30.
2. David Eddings
David Eddings is a great author, but he suffers from a flaw many authors do. They make it too easy for the hero. The stories are great, but the odds are always stacked in his heroes favour. Its not a challenge for the heroes to defeat their foes. You enjoy the great plots, the lovable characters... but eventually you feel disappointed because the hero doesn't really overcome any true trials.
3. Dave Duncan
David Eddings should take lessons from Dave Duncan. His characters get beaten up regularly. They suffer, often due to bad luck. Duncan's heroes eventually persevere however and the reader feels elated because everything comes to fruition in the climax and the baddies get their just desserts. His books are also exceptionally entertaining and quirky.
4. Margaret Weis & Tracy Hickman
Creators of the extremely popular Dragonlance series. These two work better as a team than apart. Lovable characters, interesting plot twists, realistic and heroic deaths. They bring the world alive and keep you coming back for more.
5. Charles Moffat
Moffat's works include the Lilith Bloodstone Series, the Adventures of Wrathgar, and his Wulfric the Wanderer stories. His work encompasses Dark Fantasy, Heroic Fantasy, and Sword and Sorcery. What makes his work so special? Most of his works take place in the fictional kingdom of Korovia, with overlapping storyline, the same locations used in different book, and a multitude of reappearing characters.
Read more about Charles Moffat
6. J. K. Rowling
Creator of the extremely popular Harry Potter series Rowling also has the problem of making it too easy for her heroes. She hands everything to Harry on a silver platter. Super powerful wand, invisibility cloak, the ability to fly on a broom really well... Its too much! Oh and allies to watch his back and they are always conveniently there or he is lucky. (Literally, a luck potion in the one book.) Her skill at writing however is impressive and the combined storyline of Harry Potter shows severe planning in advance so that all the details came together.
7. George R.R. MartinLongwinded but brilliant. Martin has created a fresh, new fantasy world, written great stories and unbelievable writing skills. His Song of Ice & Fire is well on its way of becoming one of the greatest fantasy series ever and now a television series, called Games of Thrones, has been made of A Song of Ice & Fire. But please, someone make him write just a little bit faster! (* He really should stop editing his own works. He needs to hire an editor who can skip over the descriptions of food and clothing.)
8. J.R.R. TolkienTolkien’s writing style may not the most accessible ever, although personally I do not dislike his elaborative style, but he is one of the greatest authors ever. The world and stories he created are fantastic and by writing The Hobbit and The Lord of the Rings he simply is the most influential fantasy writer ever.
9. Steven EriksonContrary to Martin, Steven Erikson writes like the devil himself is on his heels. I do not know another writer who writes this intense on such an epic scale. He is close to finishing his fantastic Malazan Book of the Fallen series. Hopefully, Erikson has something in mind after this.
10. Terry PratchettWhat a laugh! His stories are brilliantly written and highly entertaining. Considered to be one of the greatest British novelists, sir Terry Pratchett has already written close to forty of his very humorous and highly acclaimed Discworld novels.
11. Stephen KingDefinitely not an author of high fantasy like the authors in this list, but most of his books have fantasy elements, with The Dark Tower as best example. Stephen King simply is one of the greatest writers ever.
12. Raymond E. FeistI would not go as far as qualifying him as a genius but Feist has written many, many fantasy books and several of his series are a must-read for fantasy fans with The Riftwar Saga as his best known series.. As his books introduced me to the fantasy genre, Feist has deserved his spot in this list.
13. Robert JordanMay he rest in peace. Robert Jordan is the author of the highly acclaimed Wheel of Time series but has not been able to finalize the series before he died in 2007. I hope that Brandon Sanderson will honor him by writing a fitting end to the series.
14. Stephen R. DonaldsonHis books are not an easy read but Donaldson writes very intense and his stories have great depth and loads of morality. Known for his Chronicles of Thomas Covenant (the unbeliever), his world and characters are seen by some as rivaling those of Tolkien’s.
15. Robin HobbRobin Hobb is a great storyteller and has proven to be very good in developing strong and believable characters in her books. Especially her Farseer and Liveship Traders series are therefore highly recommendable.
16. Ursula K. Le GuinBy writing the Earthsee series, and many other novels, Ursula K. Le Guin has had a large influence on the development of the fantasy genre during the 20th century. The series has been and still is very popular and a television mini-series was even made, loosely based on Earthsee. (I personally hate her because I think her storytelling borders on the childish and lame.)
17. Terry Goodkind
He is definitely not my personal favorite but Terry Goodkind’s Sword of Truth series is highly acclaimed and very popular. It even has been turned into the television series Legend of the Seeker but unfortunately the series has been canceled.
