That’s a hard one. I know the leading candidates — The Voyages of Doctor Dolittle, A Wrinkle in Time, The High King, and From the Mixed-Up Files of Mrs. Basil E. Frankweiler. All books I loved as a child, and read, and re-read. I think it has to be A Wrinkle in Time (1963), because it did weird things to the inside of my head. I do not think I saw the universe in the same way after reading it.
winner of the 2009 Newbery Medal for The Graveyard Book