In no special order, here they are. Only a few surprises, I bet (to people of my generation anyway).
The Narnia chronicles. C. S. Lewis
Harriet the Spy. Louise Fitzhugh.
The Diamond in the Window series by Jane Langton.
A Wrinkle in Time. Madeleine L’Engle.
Half Magic. And other books by Edward Eager.
Loretta Mason Potts. Mary Ellen Chase.
The Mary Poppins books. P. L. Travers.
The Green Knowe books. L. M. Boston.
Higglety Pigglety Pop! And others by Maurice Sendak.
The Borrowers series. Mary Norton.
The Moomintroll series. Tove Jansson.
Charlotte’s Web. E. B. White.
Peter Pan. J. M. Barrie.
Alice in Wonderland. Lewis Carroll.
The Wizard of Oz. L. Frank Baum.
Bonus: Here are some children’s books and series I have loved as an adult, but that weren’t around or I didn’t discover when I was a kid.
The Golden Compass. (Northern Lights series). Philip Pullman.
The Earthsea series. Ursula K. Le Guin.
The Dark Is Rising series. Susan Cooper.
The Box of Delights. John Masefield.
The Ear, the Eye, and the Arm. Nancy Farmer.
Howl’s Moving Castle. Diana Wynne Jones.
Tuck Everlasting. Natalie Babbitt.
Unleaving. Jill Paton Walsh.
A Step Off the Path. Peter Hunt.
I’d probably also have cherished J. K. Rowling, Terri Pratchett, Neil Gaiman, Brian Jacques, and such writers as those if I had gotten them earlier. As it is, I admire them but they aren’t central to me. Nor, for that matter, is J. R. R. Tolkien, though I am fond of The Hobbit.