1) Prisoner of Azkaban
2) Deathly Hallows
3) Philosopher's Stone
4) Goblet of the Fire
5) Order of the Phoenix
6) Half-Blood Prince
7) Chamber of Secrets
Chamber of Secrets and Philosopher's Stone are the most childish of them, but Philosopher's Stone is a great introduction to the HP universe while Chamber has the least interesting plot. PoA is a favorite because it's darker than the first two but doesn't drag like the last 4 sometimes do. I was very surprized with the Sirius twist and liked the whole Marauders story
Half-Blood Prince has too much in way of hormones and "snogging" and OotP as the longest book definitely drags the most although I does have Sirius, for the last time
Deathly Hallows is the finale and I think she resolved all the plot lines very neatly. Unfortunately, I missed Hogwarts with Quidditch and other activities so it's not the top favorite for me. Finally GOB was a very good read, but nothing about it stands out enough to move it higher on the list.
That said, I enjoy them all and view as one great story