Goblet of Fire: So many plots to follow - the tournament, SPEW, Rita Skeeter, Hagrid’s ancestry, Ludo Bagman, the Weasley twins sneaking around, the awkwardness of young teenage romance, and of course, Voldemort’s return - everything inter-connected, and the Moody reveal was a great surprise, but she did a great job of dropping hints for sharp-eyed readers to pick up. Excellent example of how a mystery should be written, with great action scenes (the first and third tasks read so much better in the book than they were portrayed in the movie).
Prisoner of Azkaban: Again, a well-crafted mystery, the story of the four Marauders makes a nice emotional backdrop for the series as a whole, and the time-travel device was well-used for the ending. Also awesome was seeing Draco Malfoy and gang try to scare Harry dressed as dementors and get their asses kicked by Harry’s Patronus.
Order of the Phoenix: Like Goblet, many sub-plots to follow, and the double-struggle against the willfully ignorant ministry and the evil Voldemort meant some excitement at every turn. Highlights were the formation of Dumbledore’s Army (without Neville’s using that as a battle cry, the conclusion of Deathly Hallows would not have felt as triumphant), the introduction of Luna, the use of Rita Skeeter and Luna’s father’s magazine, Harry’s silent battle of wills against Umbridge (I liked that he constantly waved his scars from her punishment in the Minister’s face in the next two books), McGonagall’s not-silent battle of wills against her, the battle at the Ministry, the tender scene with Neville’s parents in St. Mungo’s, and, most of all, the Weasley twins’ escape.
Chamber of Secrets: I don’t know why this is so low on so many people’s lists. I found Lockhart to be great comic relief, Harry did a great job thinking on his toes (and showing faith in Dumbledore) in battle against the Basilisk and Riddle’s diary, and the series would totally not be the same without Dobby.
Deathly Hallows: Excellent wrap-up to the series, and great action in Gringott’s and Hogwarts, but otherwise, way too much wheel-spinning for me to rate it very high.
Philosopher’s/Sorcerer’s Stone: A fun introductory story, but after reading the others, you can’t help but notice a lot that doesn’t make sense. For one thing, having solvable puzzles in between the public and the stone is just dumb - what, you’re willing to let someone steal it as long as they’re skilled enough and smart enough? Hide it behind a fidelis charm or something like that. And the puzzles were one for each teacher that Harry had (and therefore the reader of Book 1 was familiar with) - but knowing now that there are numerous other courses offered as Hogwarts, the “theme” makes little sense. And they were so willing to destroy the stone after Quirrel was exposed, why so reluctuant to destroy it sooner, once they knew someone was after it? (Possible answer to some of this: they wanted to flush out the thief. But even so, why not secretly destroy it and allow a decoy to be stolen?)
Half-Blood Prince: Works as a Voldemort “origin story”, but otherwise, pretty plodding until the very end. Harry’s obsession with Malfoy began to get annoying after a while, and the identity of the “Prince” turned out to be of no story significance whatsoever (not to mention it made little sense for Snape to be so openly a Half-Blood when palling around with Death Eaters). And the romantic elements felt badly done as well.