I consider SF and fantasy different genres, so I'll make a list for each.
Fantasy
1. Tolkien - He didn't start fairy tales, but he made the genre popular and respectable. I've read through LOTR at least four times, once out loud to my children. Reading it out loud really impressed me with the beauty and variety of language he used. He has different vocabulary and style for elves, hobbits, orcs, and men. He also used his Anglo-Saxon and ancient Finish studies to create elvish and dwarvish. No one else really comes close with their quality of writing.
2. Patricia McKillip - Patricia is perhaps the most consistently, lyrically, beautiful writer alive today in any genre. The Riddlemaster trilogy is great and still makes me want to read it again. “The Forgotten Beasts of Eld” I enjoy just as much. Her recent books, “Alphabet of Thorn”, “Od Magic”, etc. are like individual gems. She is beautifully descriptive and plots her books as mysteries to be unlocked.
3. Robert Jordan - The Wheel of Time series is exasperating because of its immense length and rambling descriptions, but it has scenes that are better than anything except the high points of Tolkien’s Lord of the Rings. I love his complex plotting—in his twelve books of the Wheel of Time, there are over 2400 characters and hundreds of points of view. He has at least nine (!) main characters and typically has four or more main plot threads in each book, plus dozens of subplots. He also extensively uses prophecy and foreshadowing—many prophecies and visions from the first book are fulfilled two, three, six books later. Some are still to be fulfilled. He has created dozens of nations and cultures and maintains their consistency throughout the series. If you want complexity, he's your man.
4. Tad Williams - Many will argue he should be higher than this. Certainly his portrayal of elves in the series “Memory, Sorrow, and Thorn” is as good as Tolkien’s. When I think of elves I think of his vision as readily as Tolkien’s. His major drawback is his slow plotting. He took a hundred pages to get the plot moving in “The Dragonbone Chair.” If you're a patient reader, Tad will always reward you.
5. CS Lewis - Obviously the Narnia series is his major fantasy work. In it he blends legend and Christian allusions skillfully, while making a very entertaining an original series of stories that are accessible to the young and old. Had he specialized in this genre, I suspect he would rival Tolkien.
6. Stephen Donaldson - The White Gold weilder series is at times beautiful and then despairingly dark. Many will put him higher than I do. If you read nothing else of his, read his Chronicles of Thomas Covenant.
7. CJ Cherryh - Equally known for her SF, her fantasy is excellent. The “Fortress” series has layers upon layers of reality, moving through space and time.
8. Evelyn Nesbitt - I've only read “The Windboy” and that alone puts her on this list.
9. George MacDonald - “Back of the North Wind” is one of many great stories. He was the inspiration for CS Lewis and many others.
Gotta go. More later.
Tales of the Dying Earth is a great place to start; The Last Castle is a classic.
How about fantasy in the linguistic style of Jane Austen? There's Jonathan Strange and Mr. Norrell by Susanna Clarke. She wrote at least one novella in the same style, set in the same world, if you want to try that before embarking on a thousand page - but it's done very, very well, in language, creativity, and evoking of a strange alternate history.