Harry Potter and the Philosopher's Stone - the start of it all.
This is when Harry discovers that he's a wizard, receiving a letter from Hogwarts School of Witchcraft and Wizardry. (Ah, don't we all dream of being whisked away into the magical world?) This is when we are all pulled into the delicious, intricate world of Harry Potter; the fantastic Diagon Alley, the idea of owls delivering your posts, moving photos and portraits, actual flying broomsticks, Quidditch... the list goes on.
There's also a slight difference between this book and the other installments; J.K. Rowling writes her books in third-person perspective, but after this book, they are more narrowed to Harry's persepective. In this book, there's many more perspectives than just Harry's. And at this stage, this book can still be considered a "children's book." This aspect changes as the characters get older, and they're more aware of the darkness in the world.
This is the start of it all - this first installment is a must-have.