I was thinking back on some of my favorite fantasy and science fiction novels of my childhood. I haven't re-read these lately (except the Hero and the Crown), so I'm not sure how they hold up (except the Hero and the Crown, which totally does), but here are some I still remember fondly, for various reasons and at various ages:
The Hero and the Crown, by Robin McKinley
The Book of Three and the Black Cauldron, by Lloyd Alexander
The Search for Delicious, by Natalie Babbitt
The Phantom Tollbooth, by Norton Juster
Dealing with Dragons, by Patricia Wrede
The Princess Bride, by William Goldman
The Dark is Rising series, by Susan Cooper
A Ring of Endless Light, by Madeleine L'Engle (and other related books and series)
The Magic Coin, by Ruth Chew
The Rats of NIMH series, by Robert C. O'Brien
Charlie and the Great Glass Elevator, by Roald Dahl
Enchantress from the Stars, by Syliva Engdahl