I would say mine is the Miraculous Journey of Edward Tulane. My teacher read it to the class in third/fourth grade and I fell in love with it. It might be because I loved dolls and stuff like that and related to Abilene. I loved seeing Edward going on all of this adventures and I believed for the longest time that he was like a character in Toy Story. I think my favorite part was when he was with this brother and sister. Although many people don’t like it due to its non-kid friendly topics (abuse, manipulation, death),the book just reminds me a simpler time in my life and when I just started to realize reading was fun.
Two come to mind.
Journey to the Center of the Earth by Jules Verne, which ignited in me the flame of science fiction. I was 7. It was so mind-blowing I read everything by him in a short time and then started seeking similar things, which brought me to Clarke, Clement, Asimov and many others.
Needle by Hal clement. I read it at 10 or 11 and it was oh so good. It was the first time I’d encountered so alien an alien and that made it one of my favorite books, status it holds even today more than 30 years later.