Michael Morpurgo is fantastic - many of his books are fictional accounts set during actual historical periods (War Horse, Toro Toro, Kaspar) and his use of language is wonderful. Highly recommend him.
Narnia books - am halfway through The Magician's Nephew with DS (9) and he loves it. Such wonderful stories, inventive, humorous, thoughtful.
Alan Garner - yy to The Weirdstone of Brisingamen - was read this myself in Y6 (30 years ago!) and still read it over and over.
The Wolves of Willoughby Chase (and the rest of the series) by Joan Aiken. Another one I read at school (Y7, iirc) and I enjoyed it so much that I finished reading the book even after we'd done all the required writing for it. Read it to DD and DS not long ago - they loved it and want to hear the others now.
YY to the amazing Redwall books by Brian Jacques - who wouldn't love a mouse-monk? And yy to Watership Down (but prepared for tears if your DC are animal lovers).
Box of Delights by John Masefield is great on the run-up to Christmas - then get the BBC adaptation on DVD - we watch it each year over two or three days.
And my God yes to Susan Cooper's Dark Is Rising Sequence - love those books to death, especially the title book (second in the series) but also the first book - atmospheric, dark in places and so well written.
Can you tell I love children's books? 