Lord of the Rings is my favorite. But that's an obvious one, lol. (And C.S. Lewis' The Chronicles of Narnia series is just about as famous - though very good even if they're very much children's books.) So here's a few other favorites:
Patricia McKillip - Riddle-Master (this is a trilogy - you can find them sometimes all in one volume or separately as The Riddle-Master of Hed, Heir of Sea and Fire, and Harpist in the Wind)
Patricia McKillip - The Forgotten Beasts of Eld
Patricia McKillip - Song for the Basilisk
Alma Alexander - Worldweavers series (4 books only right now, maybe that's all: Gift of the Unmage, Spellspam, Cybermage, and Dawn of Magic)
Madeleine L'Engle - Time Quintet (series of 5 books, very famous: A Wrinkle in Time, A Wind in the Door, A Swiftly Tilting Planet, Many Waters, An Acceptable Time)
And I remember reading L. E. Modesitt's series starting with The Soprano Sorceress and liking it a lot when I was younger but I haven't re-read it in a long time.
The others I find entertaining are very well-known but some are more like "brain candy" - tasty but little substance. That applies to pretty much anything by Tamora Pierce, a good chunk of Mercedes Lackey's Valdemar (and greater Velgarth) books, and a lot of the Pern series by Anne McCaffrey. Enjoyable but they all have their flaws and aren't as solidly good to me as the ones above. (But if you're bored and haven't read them, they're a good place to go.)
