My all-time, hands-down favorite mystery story as a child was this one:
Originally written in French by Paul Berna, it's about a gang of poor French street urchins who get tangled up in a dangerous heist. I read this book over and over again, for years. I also read Trixie Beldon and Nancy Drew and The Bobbsey Twins, but they were never in quite the same league as The Horse Without a Head.
Another favorite was Emile and the Detectives.
I don't know what it says about me, but this one was originally written in German. It was, to quote Wiki, "the only one of [Kastner's] pre-1945 works to escape Nazi censorship... The most unusual aspect of the novel, compared to existing children's literature at the time, was that it was realistically set in a contemporary Berlin peopled with some fairly rough characters, not in a sanitized fantasy world; also that it refrained from obvious moralizing, letting the characters' deeds speak for themselves."
So, what was your favorite mystery as a kid?
And H/T to my friend Laura Joh Rowland, whose recent Facebook post inspired this journey down memory lane. Her favorite as a child was Mystery of the Green Cat, by Phyllis Whitney. Somehow I missed that one.