Gotta join in here. I have a number of favorites that rarely make it onto "best kids books lists." They vary in how advanced they are, but they are all ones I enjoy re-reading as an adult!

The Silver Crown
, by Robert O'Brien, is much better than Mrs. Frisby. It does have some unnerving parts, though.

Finn Family Moomintroll
, by Tove Janson. The other moomintroll books are quite dark, but this one is just a delight.

Bill Bergson, Master Detective, by Astrid Lindgren, author of the Pippi books.

The Gammage Cup, by Carol Kendall. Somewhat marred by a standard kill-the-bad-guys ending that is unworthy of how brilliant the rest of it is.

Harriet the Spy. I would have hated the realism of this as a kid, but I knew kids who adored it, and as an adult I really appreciate how great it is.

Carbonel, King of the Cats
, by Barbara Sleigh.

Diamond in the Window
, by Jane Langton.

The Changeling and The Egypt Game, by Zilpha Keatley Snyder. These two are far and away her best. The Egypt Game has one genuinely frightening bit, and The Changeling moves into teenage themes towards the end (disaffected teens committing vandalism, that sort of thing).

Louly, by Carol Ryrie Brink, author of Caddie Woodlawn.

While Mrs. Coverlet was Away
, by Mary Nash.

Haroun and the Sea of Stories
, by Salman Rushdie.

And as long as we're recommending mystery series, there's the Lord Peter Wimsey mysteries by Dorothy Sayers.