Some obvious choices for me are The Lord of the Rings books, Charlottes Web, the Laura Ingalls Wilder Books, Judy Blume books, Madeleine L’Engle books, Roald Dahl books, The Phantom Tollbooth, and so many more that I'm not listing but I wanted to spotlight some lesser know (at least from my experience as a book monger) gems:
As you guys know, I was probably scarred as a child by Tailypo but I've always had a love of being scared and all things spooky. In 3rd grade, I discovered the author John Bellairs. Most of his book were illustrated by Edward Gorey which started another life long obsession. The books are mostly separated into three main characters - Johnny Dixon, Anthony Monday, and Lewis Barnavelt who all embark on spooky mysteries. Not mysteries like, "We thought it was a ghost but it was actually just a woman that doesn't really like visitors" No, actual weird shit happening but done in a way that it's not terrifying just intriguing! Witches and weird caves! I recently reread some of them and was surprised by how they still snagged me! My favorites will always be The House with a Clock in its Walls and The Curse of the Blue Figurine. Unfortunately, the Gorey covers have been redone for a lot of them.
Honorable mention: The Chronicles of Prydain series. Perhaps that is old hat to people but I was really bummed to find out that kids do not really read those books anymore but excitedly got some to try them out. They seriously still hold up! I'm also still not convinced that Eilonwy isn't a great name for a girl.
Younger folks honorable mention: The People Could Fly. I still read this book with my mom. The author and illustrators are still my favorites.
Share your picks in the comments!