Lloyd Alexander has written a large number of fantasy books for children; not only the Prydain series (which is rather like a LOTR lite) but also the Westmark books.
I recently started reading Tanith Lee’s “Wolf” books. So far she has written “Wolf Tower” and “Wolf Star.” Rather strange, but good, books.
Kara Dalkey wrote two wonderful fantasy books set in Japan for YA – “Little Sister” and “The Heavenward Path.” She also wrote another series, not as wonderful, about mermaids.
“A Plague of Sorcerers” and “Journeyman Sorcerer” by Mary Frances Zambreno.
“Ella Enchanted” by Gail Carson Levine is a wonderful retelling of the Cinderella fairytale.
“The Perilous Gard” by Elizabeth Marie Pope is a truly classic story which treats faeries in much the same way as Tam Lin.
The various books about Bordertown by Will Shetterly (and Emma Bull?)–Nevernever, Elsewhere, Finder.
Patricia Wrede’s books–all of them, but particularly the Enchanted Forest Chronicles, starting with “Talking to Dragons” and the Mairelon books, starting with “Mairelon the Magician.”
The Wren books by Sherwood Smith, starting with “Wren to the Rescue.”
Some classics for younger readers: anything by Edward Eager or E. Nesbit.
The Mennyms series by Sylvia Waugh, starting with “The Mennyms,” about a family of life-like dolls an old woman created and how they survive after she dies.
“The Thief” and its sequal “The Queen of Attolia” by Megan Whalen Turner. I especially recommend “The Thief,” which was a Newberry Honor book.
There was an entertaining YA book about a young girl who goes up this mountain for some reason and various obstacles present themselves. There’s a field of jagged rocks, which her tears melt, and rather disturbing colored snow with various unpleasant properties, all made by this evil witch. All ends happily, of course. Can anyone remember the title/author? It’s bugging me now. If anyone can, it was a good book so add that too.
Everyone has seconded DWJ already–but let me add another vote. Simply the best YA fantasy writer out there, in my opinion.