Favorite one of the series? Honestly, I don't know. They're all fantastic and they kinda get meshed into one giant book because I'm reading them all one immediately after the other..
Stefan was an awesome addition to this book. I fell in love with the story of him and Vic as younger kids in Camp Hell, and their romance. I'd like to read that mini-story!
Another home-run though, serious. These books actually make me focus more on the story than the romance (cough*sex*cough) and I think that's great! The plots are interesting and exciting!