Alexander, Lloyd — his Prydain world was neither too big nor too complex for a growing reader; instead, it showed me the value of vibrant characters even in the midst of the unearthly and fantastic.
Lewis, C. S. — first the Narnia series (of course) when I was small, then Screwtape Letters and the Space Trilogy when I got older; and the essays when I was an adult.
Tolkien, J. R. R. — my idea of the ultimate world-building author; I mean, not many fantasy writers are capable of creating entire languages and cultural mythologies just for one cycle of stories.
L’Engle, Madeleine — A Wrinkle in Time put extra wrinkles in my youthful brain. What more can one say?
Sayers, Dorothy L. — yes, she wrote cozy mysteries instead of fantasy, but her word-play remains delightful after multiple readings and her meticulous research is unparalleled in her field.
McCaffrey, Anne — her Pern novels gave my pre-adolescent self a view into truly mainstream fantasy, as well as suggesting myriad different uses for dragons.
Pratchett, Terry — discovered well into adulthood, upheld as a paragon of satire (for those who see virtue in sarcasm); my favorite characters remain the Wee Free Men. Och! (Second-favorite: the tyrant Lord Vetinari. That may be because I sympathize with him in his desire to keep things well-regulated– even crime. )