Even as the weakest link in the Harry Potter chain 'The Chamber of Secrets' still works and, my goblet being half-full, it's the shortest!
I said I wouldn't focus on the wizarding-world-makes-no-sense-at-all problems that occur to me whenever I read these books, but 'Chamber of Secrets' is the only Harry Potter where its less the magical thinking and more the actual lack of thought at all from the characters that causes problems. The general silliness and lack of logic that enlivened Harry Potter and the Sorceror's Stone and the other books gets out of hand, even if Moaning Myrtle and Dobby turn out to be surprisingly important characters. Lockhart, Dumbledore hiring Lockhart, flying/wild cars, giant talking spiders, Malfoy jr. (even in private all he thinks about is Harry Potter? Slash-fiction COMMENCE!), Malfoy sr. being such a clumsy idiot about getting Dumbledore suspended....The main plot of Slytherin's Heir is good but robbed of malice when one thinks of how small the "body count" is and that that number includes a cat. (Heir, psst. you can do better.)
On the other hand the chemistry of Harry, Hermione and Ron only gets better and there is some important foreshadowing: we get to know the Weasleys, the plight of house elves, the Heir reveal....So maybe there isn't a lot of good that comes to mind, but I still like it as a breezy kid's book even if not as part of the Harry Potter series.