I'd probably have to say it's The Wizard of Oz, which I first read when I was about five. It was a librarian's recommendation to my mother, since she'd been trying to find material that was challenging enough for an early reader while still being age-appropriate. Thanks to that book, I landed squarely in the realm of fantasy novels, and never really left. ;)

(The year before, my favorite book in the WORLD was Charlotte's Web, which also deserves a shout-out. I read that book literally to pieces. Scotch tape was involved.)

US.