Core children's and YA titles

We are spending our book budget for our church library this month and I would appreciate any suggestions on fiction for young adults and children that you think are must haves. Picture and chapter books are fine.

Philip Pullman's His Dark Materials series has got to be the very best YA fiction (and I use the term advisedly, because these books are great for adults, too!) on spiritual matters in a fictional setting that I have ever come across. The trilogy is:
The Golden Compass
The Subtle Knife
The Amber Spyglass

I always thought Pullman's article, "The Republic of Heaven" in The Horn Book (November/December 2001) would make a terrific sermon. Excellent suggestion--thanks.