I wasn't messing around with these prompts, was I? Favorite DWJ Book or Series tends to be, for me, whichever one I just finished reading or re-reading. I can't imagine choosing just one. So, here are my Top Five --
House of Many Ways
Dark Lord of Derkholm
Year of the Griffin
The Pinhoe Egg
These are all magic stories and, strangely, almost all featuring special houses. They're mixed between male and female protagonists, so that's good. There is also very little direct parenting in these books (except the awesome Derk and Mara), which makes sense considering how horrid all of Diana's parents usually are. Three have griffins, which I love. And all of them are perfect DWJ stories.
What is your favorite book or series?