I was just trying to work out the other day what my favourite DWJ was. (I've been doing some re-reading.) I, too, don't think I can pin it down to a single favourite. Fire and Hemlock is definitely up there; it's probably the one I've reread in its entirely the most times, and the one that stirs the deepest emotional response. Howl's Moving Castle is definitely another favourite, and so is The Lives of Christopher Chant, which was the first of hers that I read. (I wish I'd read Charmed Life first, though - another one I love.) I'm also very fond of Dogsbody, and although I don't normally like her adult books as much, I do like Deep Secret very much, too. While I wouldn't put it in the ranks of all-time favourites, I enjoyed Conrad's Fate a lot, too; I just love teenage Christopher.

But if we're talking about books of hers that I most often open up, that would have to be The Tough Guide to Fantasyland, which I use as a reference book. :-)