Depends at what age you'd ask me that.

Harry Potter 4 was the first book I binged when I was a kid. Lord of the Flies was the first book that made me feel weird emotions, and I liked it. Snow crash is the book that made me think "Fuck, how can one write a book like that ?" and therefore started what I hope to be a lifelong hobby. I still think of Flatland from times to times, as I'm jealous and amazed of the brain of its author.