7. The Chamber Of Secrets
Into the main Rowling series proper now, and Harry Potter and the Chamber of Secrets is the 'worst' of those, which really just means it's not quite as great as the others.
Chamber of Secrets does have a lot going for it: the flying Ford Anglia is wonderful, it has a captivating mystery, and it nicely builds on the characters and relationships introduced in the first novel. It's also sh*tting terrifying, if you're scared of spiders, anyway.
One of the problems with Chamber of Secrets is that it introduces a number of key elements, but we don't know that at the time. The Whomping Willow is just a random magical tree; Polyjuice Potion a fun trick; the major villain of this story is just a bloody diary?! Without the later context, these narrative elements aren't quite as strong.
It also suffers from largely being a re-tread of the first book, sticking to the same tried-and-trusted formula, which means it's a little thin. Also a few important characters are quite grating in this one (Dobby, Lockhart, Myrtle...), which adds up to it being the weakest of the original seven.