One of my favorite things about the series is how the books pair up. There are themes that weave throughout all of them as well as the larger narrative, but in the details the books each seem to have a “partner” in the series - except for GoF. (The pairs I think of are 1 and 7, 2 and 6, 3 and 5.)
My favorite two are CoS and HBP, but I can’t pick between the two. It depends on the day.

Looking back at CoS, I see so much that set up the end of the series that I completely missed on the first read (granted, I was 14 when I first read it and I’ve grown up a bit since then.) I am big on character development, and Harry grows up in a big way from the first book to the second.
</English lit nerd>