I probably read/buy more children's books than adult books, as they are often more original and interesting, and deal with growing up, which I think we all do all our lives.
My all time favourite is Diana Wynne Jones, although her more recent books haven't really got quite the same magic as the ones she wrote in the 70s/80s. It's sad to hear that she is dying, but she must be quite old (or started writing very young).
I love, own and have read more than a few times a lot of the authors mentioned here (Joan Aitkin, Susan Cooper, Angie Sage, Philip Pullman, Garth Nix, Michell Paver, Cynthia Voight, Robert Cormier (although scary stuff), Anne Fine (both her childrens books and adult novels), Michael Morpurgo, Eoin Colfer, David Almond, Malorie Blackman) although there are a few I've not heard of and will be checking out.
Other authors I've enjoyed which haven't been mentioned are:
Sarah Prineas
Charlotte Hapte
Jenny Nimmo
My only problem now is getting my children to read them as they are quite suspicious of books I recommend, even though I have several times got it right - ds is addicted to Artemis and is racing through the Mr Monday series, whilst dd and I fought over the latest Septimus book (although it was a bit disappointing)
We don't do it the other way around so much, they both love the Warriors series, and I don't like anthropomorphism very much so I've not touched that really. But dd recently read all the Twilights, and I'll read those just to make sure we have the right sort of conversations (although dd at just 10 is at that horribly mature pre-teen stage and tells me that Bella is a poor role model who made bad choices, so I'm not too concerned!)