I've always loved Howl, and I have a deep soft spot for Sempitern Walker. But the major places in my heart, oddly enough, seem to go to Hathaway, Torquil, and Erskine (Archer's Goon), especially Hathaway, who is dealing with the godsawful mess of that family with grace, humor, compassion, strength, and an impressive means of getting as far away as possible. Torquil is such a brat I can't help but like him, and I respect Erskine's intelligence greatly.

Also, I identified so strongly with Polly from Fire and Hemlock that I can't even tell if I liked her or not. That book is an essential part of my worldview; probably my three favorite DWJs are the ones I've mentioned characters from in this comment, though I feel that The Ogre Downstairs is often underrated and that Eight Days of Luke takes some kind of award for causing me to like it even after I guessed the premise from the title.

The only DWJ I actively hate is Dogsbody, because I read it way too young (seven) and it hurt very badly.