LeGuin’s Earthsea books (the first three) are still excellent. I recently read Tehanu for the first time, and went back to re-read A Wizard of Earthsea, The Tombs of Atuan, and the Farthest Shore, which I first read when I was about 8 or 9. Her earlier writing was much, much better. More resonant, great use of language, and no blatant feminist screed embedded in the story.
The books of the True Game, by Sheri S. Tepper are very good. I’m currently re-reading the three books about Mavin Manyshaped that I came across in a used book store a couple of weeks ago. I appreciated them as a kid for the adventure, and I’m appreciating them now as well-done stories that tap into the same kind of depth that fairy tales have.
I’ve read the Prydain Chronicles a few times, and they still held up well as of a couple of years ago. Suppose that’s not all that surprising considering that they’re a retelling of the Mabinogen, but still, they’re a good retelling.
I loved The Hobbit as a kid, read The Lord of the Rings when I was a bit older. Obviously, Lord is good enough to spawn serious scholarship, but Hobbit is by any standard an excellent book in its own right.
Just about anything by Heinlein. It took me from childhood to early adulthood to track down and read most of what he’d written, and I’ve read some of his books and stories multiple times. Even his “juveniles” are darn good for an adult reader.
Most of Madeleine L’Engle’s stuff, though now that I’m not a Christian anymore, some of the intent falls a bit flat. The writing and message are still good though, even if you’re an atheist.
I actually didn’t read the Oz books when I was a kid. I read the first one a couple of years back and actually wasn’t all that impressed. I might have liked them when I was very young, but was already reading and appreciating adult-oriented literature when I was in grade school.