When I used to be a member of a swedish Harry Potter fan community, before the website got shut down due to illegal use of the hogwarts name, I was often confounded that pretty much everybody would put Harry Potter and The Chamber of Secrets at the bottom of their list when asked how they'd rank the various books. The other books had different ranks depending on the person but CoS almost always got ranked last.
As I always thought that it was at least superior to Philosopher's Stone I never quite understood why people ranked CoS as the least great one. It wasn't until my recent read through of it, now that I can see it with more experienced eyes, that I can understand why this is/was the case.
I still don't agree with the assessment that it should ranked last though.