Book recommendations

I hope this isn't too close to the nominate your favorite YA book thread. (If so I apologize) What I am looking for is a list of books that you (everyone here) would recommend to someone who is writing YA or middle grade novels. What books would be helpful in illustrating qualities that are desirable in a piece of fiction targeting this age group? (Man, that sentence was awful...please forgive me) Which books are a good example of what not to do? You don't have to limit it to one book, the more the merrier. I'm just looking for reasons why you liked it or didn't like it. Also what you thought the books strengths and weaknesses were. I'm raiding my son's bookshelves and will come back with my own opinions on some of them later. (Though my opinion, because it is coming from a complete novice, isn't worth diddly)

Thank you all, this is great - I am reading the His Dark Materials trilogy right now, how funny you should mention him . I like it a lot, but I'm not as blown away as I was expecting...then again I haven't finished the last book yet. It was interesting because I picked it up on a whim and it really touched on some things I was struggling with in my WIP. Actually I was struck by the fact that it didn't attempt to explain every little thing, it just kind of stated it as fact or gave a comfortable fuzzy (fuzzy as in vague) description. I deal with the many worlds theory in my WIP but not in quite the same way-(not on the same level) I like how he explained the subtle knife - that was my favorite of the books so far. Fluffy wizards? Hehe I'm imaging my son's story about cat wizards now . I like things a little darker, but unicorns have their place .
Thank you all for the great recommendations. I should mention my main interest is in the fantasy spectrum, not that I don't like other books, I just read and write (mostly) fantasy.