Several already mentioned (including Goodnight Mister Tom which is an absolute must-read, although it's pretty harrowing), but I'd also mention Jacqueline Wilson's books. She's an absolute genius for first-person character-driven narratives, although I only read them as an adult.
The Weirdstone of Brisingamen and other books by Alan Garner - best kids fantasy out there; also Susan Cooper's The Dark Is Rising, simply butchered by the movie.
Watership Down is pretty amazing.
In more recent years I've looked at quite a few young adult books my kids have got into - the Charlie Higson zombie series starting with The Enemy is pretty terrifying; I like Artemis Fowl too, and my daughter loves Hunger Games but I haven't read it yet.
I read all kinds of crazy things as a kid, though. 'Fattipuffs and Thinnifers', for example - a social satire about a world divided into a nation of fat people and a nation of thin people. 'Sponge, X and Y' - an adventure story about three fish and their friend the Hobgoblin (who has magic boots that he can use to turn monkeys into Supermonkeys). 'Bottersnikes and Gumbles' - a series of books about little squashy creatures who keep getting captured by big fierce creatures and kept in jam jars. I devoured that crazy shit.
I was also a fan of E Nesbit - I still think she was an incredibly revolutionary children's author. 5 Children and It was great, and a lesser-known one called The Enchanted Castle.