I could have sworn that I replied in this thread...guess I'm hallucinating again.
Anyway...I didn't read a lot of YA or children's books (to the despair of the library ladies, who looked askance on it when I started using the adult section of the library when I was quite young), but a few stand out.
On the SF side, first, Heinlein's
Red Planet was probably the first proper science fiction that I read on my own (as opposed to being given it by my father), although I had been exposed to it on film virtually from birth.
However, most of the books in this category that made an impression on me were not science fiction or fantasy. As others have said, I was a big Nancy Drew fan. I didn't read all of them, as my library did not stock them, but I was given a few of them as gifts and I treasured them. I also loved the Laura Ingalls Wilder books. I still read those from time to time. And there was something called
Susie and the Ballet Horse, whose author I cannot remember. It was about a little girl who went off to ballet summer camp (I was studying ballet at the time, so the subject matter attracted). She fell, however, and broke her leg. With nothing else to do, she started exploring the area around the camp - cast, crutches and all - and discovered a nearby farm where the owner was training a Lippizan dancing horse, where she begins to help with that.
Another YA book that was a formative influence in my life was
The Romance of Archaeology by Anne Terry White. I had discovered archaeology on a visit to an ancient Native American site in Arizona...there was this mummy, and that was it for me...and so I grabbed this book and absorbed it and have been in love with archaeology ever since.