Finding great fantasy books isn't as easy as it ought to be, especially in the post-Potter era, in large part because so many of them belong squarely in the realm of genre fiction. That is, most fantasy books follow the established conventions of the fantasy genre in a such a formulaic way that they satisfy only those who are ardent fans of those conventions. In general, I find that children's fantasy tends to be more interesting and less bound to the conventions of the genre, as well as better written and without the highly fetishistic sex scenes prevalent in adult fantasy. They also tend to be much funnier - a necessity when the genre taken straight is so easily self-parodying. As a result, most of the series on this list are considered children's literature, though I find they improve with age.
6. Westmark Trilogy - Lloyd Alexander
(Westmark, The Kestrel, The Beggar Queen)
Alexander's trilogy is like Game of Thrones-junior - slightly less violent, much wittier, and decidedly shorter. Theo, a printer's apprentice, is on the run after his master is accused of violating strict censorship laws, and joins the charlatan Count Las Bombas, his servant Musket, and street urchin Mickle. All of them are drawn up into a complex political situation that promises to escalate into war. Although written for young adults, Alexander doesn't shy away from graphic depictions of violence and its moral consequences and there are no clear-cut dichotomies between good and evil.
5. The Once and Future King - T. H. White
(The Sword in the Stone, The Queen of Air and Darkness, The Ill-Made Knight, The Candle in the Wind, The Book of Merlyn)
This series based on Arthurian legend is the most colorfully imagined interpretation of the stories of Arthur and his knights of the Round Table and White's version of Merlin is definitive. Wonderfully witty and dazzlingly erudite, The Once and Future King is a mixture of medieval saga, meditative character study, and political allegory with more than a small share of romance and magical education. The Disney animated film, based on the first book, isn't a straight-forward adaptation, but is nevertheless delightful.
4. Prydain Chronicles - Lloyd Alexander
(The Book of Three, The Black Cauldron, The Castle of Llyr, Taran Wanderer, The High King)
This fabulous series, partially based on Welsh folklore, follows Taran, an assistant pig-keeper, a creature of unknown origins named Gurgi, a bard, and a princess, as they fight the Horned King and the undead hordes born from the black cauldron. Alexander once again creates characters grappling with the moral implications of violence, as they fight to reinstate peace in Prydain. Often unexpectedly funny, the Chronicles offer everything one could wish in a fantasy series. The Disney adaptation is an absolute nightmare of cliched plot and two-dimensional characters - definitely skip it.
3. A Song of Ice and Fire - George R. R. Martin
(A Game of Thrones, A Clash of Kings, A Storm of Swords, A Feast for Crows, A Dance with Dragons, two yet-to-be-published volumes)
Martin's monolithic opus is extraordinary for many reasons. Firstly, these books are incredibly long and complicated, with dozens of characters, and yet there are no significant plot-holes and most of the characters are complex, evolving beings. Secondly, the whole concept of good and evil goes straight out the window, especially after the first book. In no other fantasy series are the politics so circuitous and elaborate that it is absolutely impossible to designate who the "good guys" are (and when you think you have, there's a plot twist coming to prove you wrong).
2. Harry Potter - J. K. Rowling
(Harry Potter and... The Sorcerer's Stone, The Chamber of Secrets, The Prisoner of Azkaban, The Goblet of Fire, The Order of the Phoenix, The Half-Blood Prince, The Deathly Hallows)
Rowling's series was and is a literary phenomenon that will never be repeated, despite the publishers' many attempts to do so. Rowling's robust sense of humor never fails, even when Harry is in the most dire of circumstances and the series delves into profound issues without the slightest pretension or grandiosity. The wizarding world is one of the most fully imagined of all fantasy worlds, inviting, imperfect, messy, and magical. For all these reasons and quite a few others besides, the Harry Potter books are infinitely re-readable, and I should know, having read them about a dozen times.
1. Chronicles of Narnia - C. S. Lewis
(The Magician's Nephew, The Lion, the Witch, and the Wardrobe, The Horse and His Boy, Prince Caspian, The Voyage of the Dawn Treader, The Silver Chair, The Last Battle)
It's difficult for me to write about the Chronicles of Narnia without descending into a morass of ecstatic superlatives, given that they are my favorite books of all time. Quibbling about Christian allegory often gets these books unfairly set aside (all the more ironic today given that Harry Potter can also be easily read as a Christian allegory), but the Chronicles have much to offer to any reader whether or not he or she is Christian. All the characters, including the children, are wonderful, the anarchic mix of mythologies is delightful, even the illustrations are lovely. But avoid at all costs the abominations Disney has the gall to release as film adaptations because they are truly nauseating.