As usual, these are in no particular order and may be subject to change at any given moment. (Favorites are rarely forever, and change as we do.)
The Horse and His Boy by C. S. Lewis
The Lord of the Rings by J. R. R. Tolkien
The Queen of Attolia by Megan Whalen Turner
Watership Down by Richard Adams
Jonathan Strange and Mr. Norrell by Susanna Clarke
The Sword in the Stone by T.H. White
This version (the original, almost stand-alone version, as contrasted with the shorter, altered version included in The Once and Future King) is what Disney based their loosely adapted film upon and what sparked my interest in King Arthur and the stories around him.
Really, my fascination has almost always been with Merlin, the wizard who tutors Arthur and guides him toward his destiny as king; but Arthur is just as often a background figure in Arthurian stories as he is a main character, so I don't feel bad about that.
Harry Potter and the Deathly Hallows by J. K. Rowling
The Bartimaeus trilogy (The Amulet of Samarkand, The Golem's Eye, and Ptolemy's Gate) by Jonathan Stroud
I know, I said I was only picking one book per series. Unfortunately, I can't pick just one of these three. Each has its strengths and weaknesses, but as a whole it's one of the best fantasy stories I have ever read. The character arcs of the three main characters (Bartimaeus, Nathaniel, and Kitty) are simply astounding. Stroud begins with the simple premise that the ruling class of magicians from the old stories is not so much a stratum of benevolent guardians as it is a corrupt coalition of power-hungry individuals, and the story follows that premise as a young boy (Nathaniel) is swept up into this glamorous world of power and intrigue. The witty, sarcastic, and sometimes heartfelt narration from Bartimaeus makes this world come alive.
Sabriel by Garth Nix
The first in Nix's Old Kingdom series, this book follows the story of a girl named Sabriel as she tries to fulfill her father's role as the Abhorsen, a necromancer of a different sort. Instead of summoning and enslaving the dead, Sabriel, her father, and their ancestors use magic bells to keep the dead in the grave. The concept alone drew me into the book, but Nix's keen writing made me stay. His characters are real, complicated people who sometimes want nothing more than survival, but who feel the call to do more with their lives. While his recent prequel, Clariel, didn't quite live up to my expectations, everything else that I've read in this series* has delighted me.
*Sabriel, Lirael, Abhorsen, "The Creature in the Case" (in Across the Wall)
Inkheart by Cornelia Funke
If there's anything a book-lover loves more than books, it's a book about books. While the idea of being transported into a book isn't the newest idea (The Neverending Story did that in the '60s), Inkheart has a personal power to it. The fact that Meggie and her father are at risk because of Mo's affinity for books makes the tragedies they fight through that much more meaningful.
This series also includes one of my favorite characters ever, Dustfinger, whose journey toward redemption is one of the most satisfying things in literature.
This is only the tip of the iceberg. I won't let myself include honorable mentions because the post would go on for pages.
Let me know in the comments if I missed any of your favorites.