The towering greats:
Elidor, The Owl Service, Alan Garner
The Dark is Rising Sequence, Susan Cooper
The Earthsea Trilogy [post Dec '15], Tehanu, Ursuka K Le Guin (there are also other amazing follow-ups, but they came much later)
The Chronicles of Prydian, Lloyd Alexander
The Dalemark Quartet, Diane Wynn Jones
The Song of Wirrun Trilogy, Patricia Wrightson
Other wonderful, and often influential books, well worth seeking out:
The Prince in Waiting Trilogy, John Christopeher
The Whispering Knights, Penelope Lively
The Young Wizard Sequence, Diana Duane
The Changes Trilogy, Peter Dickinson
The Hero and the Crown, Robin McKinley
Red Moon and Black Mountain, Joy Chant
The Hounds of the Morrigan, Pat O'Shea
Marianne Dreams, Catherine Storr
Fintan's Tower, Catherine Fisher [post Feb '16]
The Redwall Series, Brian Jaques
Beadbonny Ash, Winifred Finlay
The Wind On Fire Trilogy, William Nicholson
Charlotte Sometimes, Penelope Farmer
The Snow Spider Trilogy, Jenny Nimmo
Boy in Darkness, Mervyn Peake
The Ghost Drum, Ghost Song, Ghost Dance, Susan Price
The Troy Game, Jean Morris
(I am sure that I will add to this list over time as I read more of the great fantasies from the second half of the twentieth century.)