Oh gods. I have so much trouble with favourites questions. They usually end up with me just kind of yelling “I’m poly for a reason, dammit!” :P
That said, I started reading pretty early, and it was a good way to distract myself from a lot of the stuff happening in my house when I was a kid, so I read a lot, back then, but my memories of back then aren’t great also, so I don’t really have any like, deep emotional attachment sort of books from actual childhood? :/
I read a metric butt-tonne of Stephen King when I was a child (which is probably for the best, since I find him difficult to read these days), though never IT, because clowns.
I remember a somewhat intense fondness with The Babysitter’s Club series, and I actually think I had a fairly large collection before I moved out of my parent’s house. It might even still be there, given their tendency to keep my things, but I don’t really want to go looking.
Also Enid Blyton’s Mallory Towers series, and I still want to find copies to re-read to figure out if it’s in fact a lot more or a lot less queer than I remember. :P