I read a lot of science fiction and fantasy when I was younger, so I was surprised when I was thinking about this list at how few books were on it. The other thing that is interesting is how many books I just remember a few details from, but not anything useful like a title or author.
Half Magic and the whole series, by Edward Eager: My favorite is Knight's Castle, although I suspect I would have enjoyed it more if I had ever read Ivanhoe...
Mrs. Piggle Wiggle by Betty MacDonald: I'm not sure when it happened, but I now identify with the parents rather than the kids.
The Seven Citadels by Geraldine Harris: I came back to the Jr. High library to check this out even after I moved on to the high school.
Girl with the Silver Eyes by Wilo Davis Roberts: I always wondered what would happen if I had ESP and other "special" abilities.
The OZ books, by L. Frank Baum: I read almost all of these (all the ones I could find in the library, rather). My favorite is Tik-Tok in Oz. I have actually never read the first one on which the movie was based. I have heard that reading them with political commentary about the time is actually very enlightening: the various characters represent philosophies and characters in politics from the time.
Giladen, by Emily Buchwald: A story of a rabbit who helps find his friend's true identity. Takes place in medieval Europe (not a specific country).
The Search For Delicious, by Natalie Babbit: One of those books I remembered the plot from, but thought I must have dreamed it.
The Wonderful Story of Henry Sugar and 6 more, by Roald Dahl: I like this one better than any of his other works, although my sister might put in a vote for "The Fantastic Mr. Fox".
The Pyradin Chronicles, by Lloyd Alexander: A good "King Arthur" story. The Remarkable Journey of Prince Jen is also very good, gives a taste of china.
The Once and Future King by T. H. White: The ultimate King Arthur story. I will also mention Taliesin by Stephen Lawhead---the first book in the series is my favorite, about Merlin's mother and father. There was also a series of 3 books written from Merlin's perspective---I can't seem to remember the title or author, though...
The Phantom Tollbooth by Norton Juster: It definitely expanded my idea of learning and the various types of knowledge in the world. See also The Dot and the Line: a romance in lower mathematics. One of the books that got me into mathematics, I think.
Tolkien: I received The Hobbit when I was about 10 from my uncle, but I didn't manage to read it for long after because it was too "scary". I was missing a lot.
The 13 clocks by James Thurber: This story is spooky, but also as funny as anything that Thurber has written. Well, maybe "Leave your mind alone" is funnier, but the 13 clocks is fun.
The Chronicles of Narnia by C. S. Lewis: My favorite is The Voyage of the Dawn Treader, Michael likes (and can almost recite) The Horse and His Boy. Eleanor loved having these read to her, although I think our enjoyment of them contributed to her enjoyment.
Hmm, the list has expanded since I first started writing. Things jog my memory and one book lead to another... What a fun list to remember. There are other one's I'm not sure fit on this list: "the Moon is a Harsh Mistress," for example, or "Ring World".
There is one book I'm looking for: it is about a post-nuclear war underground city which is falling apart. Everything you need is rationed out in daily doses given by computer. The population is kept subdued by mind-numbing education and by illiteracy. Two kids, one from above and one from below, know how to read and use that skill to escape to the 20th level. I think it was something like "Tomorrow's Children?" "Escape to tomorrow?" The last name of the author was from the 2nd part of the alphabet (based on where I remember the book being located on the library shelves). Anyone know what I'm talking about?