Spinner's End was interesting, in that we see Snape away from Harry for once, and also see him (and JKR) tackling the questions about him head-on. After the Burial was a surprising chapter in many ways -- Harry's being very 'adult' in this (and treated as such by both teachers). All the books do have their flaws, but OotP to me feels 'smoothest' (with the possible exception of CoS which has the most self-contained plot, I think). Glad to see someone agrees on 9. :)