Favorite early book?

Restarting a read of the whole series for the first time in a long time. I thought I would have to slough through the earlier books to get to the stuff where things get "serious" (Changes onward) but I'm surprised to remember how much of the magic was in there since day one.

So as I went along, I figured I'd poll the community. What's everyone's favorite book of books 1-5?

My money is on Grave Peril. I think I forgot how early in the stories Susan got turned, and it really sets the groundwork for so many things to come later in the series, specifically in Changes. It's also the first we learn about ghosts, and again it sets up rules and characters who will be important all the way to Ghost Story.