These are mostly fantasy, but uniformly excellent:
The Dark is Rising series by Susan Cooper (especially the book "The Dark is Rising") - forget about the abysmal movie they recently made and that the author hated.
The Earthsea books by Ursula K Leguin (starting with "A Wizard of Earthsea" and which she just keeps on adding to, getting better and better as you grow older with them.)
The Chronicles of Narnia by C.S. Lewis (forget about the whole "Ooh, but it's Christian!" - when you're a kid reading them, that doesn't come across and they're great reading.) Read them in publication order starting with "The Lion, the Witch, and the Wardrobe" (not the newfangled and completely WRONG order.)
"Charmed Life" by Diana Wynne Jones (very fun and you can see where Rowling stole many ideas - really, many of the books I'm listing were sources for her.)
"A Wrinkle in Time" by Madeleine L'Engle
"The Riddle-Master of Hed" (and 2 sequels) by Patricia A. McKillip
Possibly "Master of the Five Magics" by Lyndon Hardy (I really liked it, the sequels aren't quite as good.)
Of course, "The Hobbit" by J.R.R. Tolkein
And I imagine some people will recommend the "His Dark Materials" trilogy by Philip Pullman, starting with "The Golden Compass". The first book is OK, but I feel they go downhill until they're just anti-Christian rants.
Then, as your kids age, if they're liking the fantasy, I'd say the Fafhrd and the Grey Mouser books by Fritz Leiber, starting with "Swords and Deviltry" would be good. But I'm thinking they're a bit too old for pre-teens.
I've loved most of these books for years and re-read them periodically. I hope your kids like them, too!