The Egypt Game, Zilpha Keatley Snyder (1966): Rich in detail, a paean to imagination. While I was never that into Egypt, I recognize the obsessive play-as-construction; my sister and I used to spend hours building our Barbies houses, decorating them, dreaming up backstory, and then suddenly it was time to clean up, and we'd protest: "But we haven't even started playing yet!"
The Westing Game, Ellen Raskin (1978): I remember reading this one out loud to my parents and sister, every night after dinner for several weeks. Strangely, I had never owned a copy; my fourth- and fifth-grade teacher, Libby Eaton, had a classroom library, and I'm guessing I checked this one out nearly as much as I did D'Aulaire's Book of Greek Myths from Earhart Elementary's main library.
What struck me this time around, in between the puzzle, is how adult the book is--not the hijacked meaning of "pornographic," but "mature," "wise," even. It deals with suicide, losing a child, guilt, homesickness, disease, race, the pain of not living up to your parents' expectations; frustrated dreams and disappointments galore. But it's not didactic at all. It's fun, and full of unlikely friendships and wordplay. Among all of these childhood favorites, it's the one I've reread the most, and the one I'll keep rereading; the one that least deserves dismissal as a "children's book." As if children aren't real people.