Of course, there are lots of criteria that might be used to assess each book, so let's consider a few in turn, and like my post on Indiana Jones, the answer might become stunningly obvious. And, of course, if you have not read all of the books, be prepared for spoilers.
- Most moving story: Harry's discovery of his identity and of his parents in the first book is really quite touching, but Deathly Hallows is probably the most moving, not just for the many deaths, but also the maturation of Harry as leader, confronting a very, very dangerous world.
- Best Subplot: The Ron/Hermione relationship in Half-Blood.
- Least Original/Least Necessary Plot Point: When Harry Spider-man's Ginny saying that he cannot be with her at the end of Half-Blood. The Weasleys are targets and I don't think Voldemort would distinguish between ex- and current girlfriend. A small taint on a kickass ending.
- Best Action: Got to be Deathly Hallows, since there are so many great sequences from start to finish.
- Best Humor: The romantic plots in Half-Blood come pretty close, but I would still vote for the Fred and George stunts in Order.
- Most Agonizingly Dumb Mistake: Wearing the locket and continuing to wear the locket in Deathly Hallows is actually second to the unexplained non-use of the magic mirror in Order of the Phoenix. The first mistake temporarily split the team, the second killed Sirius. In most Star Trek episodes, they had to take the transporter out of commission and out of the plot equation. Here, JK introduces a device that is ignored until it is handy in the last book. She didn't make too many mistakes, but this is a big one.
- Best Kiss: Close call between Harry and Cho in Order and Harry and Ginny in Half-Blood. Since I had long predicted the Harry and Ginny pairing (not that it took a Ph.D. or anything), I have to go with the latter.
- Best Non-Voldemort Villainy: The deft effort by Barty Crouch Jr. in Goblet of Fire, actually teaching Harry a great deal about fighting the dark arts just to get him in a position where he can be used to resurrent Voldy.
- Best Death Scene: Deathly Hallows--If you don't know who I am referring to, you don't know me.
- Worst Stalling: 40 days and 40 nights and then some in the forest in Hallows so that JK can stick to the school year theme of the previous six books sans school.
- Best Defense the Dark Arts Teacher: A close call between Professor Lupin in Azkaban and Harry in Order of the Phoenix. The sequences in Phoenix where Harry is asked, then reluctantly agrees to teach, and then the sequences where he instructs his friends are some of the very best in the series--for heart, for humor, for touching this professor's soul (if I have one). But Harry can only do this because of the education he received by Lupin (perhaps a dodge to give some kudos to one of the earlier books since my lousy memory leads me to favor the newer ones).
- Worst Teacher: Gilderoy Lockheart. A wonderful blowhard, played perfectly in the movie by Kenneth Branagh, in Chamber of Secrets. Sad to say that he did a worse job than someone posing as Professor Moody, but often incompetence can do more damage than deception [you can guess which category Bush/Rumsfeld are in]. Umbridge does not count here since she was more of an administrator than a teacher, and did inspire the students to study hard, just not the way she wanted them to study (by taking Harry's informal class).
- Most funky plot device: As Dan Drezner pointed out in his blog, it turns out that property rights plays a big role, perhaps as big as LOVE, in Deathly Hallows. Who had which wand when and by what means was it taken? Either wow or oy.
- Most Fun: Sorceror's Stone and Azkaban. Order comes pretty close with the pranks against Umbridge, her wonderful comeuppance, Harry's DA lessons. But books four through seven end with deaths, which take a bit of the fun out of them. Both Stone (Philosopher's Stone for the non-US crowd) and Azkaban are pure joy--Harry and his pals win and win decisively in each. Indeed, Azkaban's ending saves a cool animal (I am a big grif fan) and gives Harry some new family for a few books.
- Best Setup: Stone. It does an amazing job of setting up the rest of the series. I wish my students could write introductions that could set up their work half as well (sorry).
My inclination is go with Hallows because, for all of its faults, it was clearly the toughest to write, with the greatest expectations. It was the most enthralling, despite its flaws, as JK and Harry took the greatest risks and survived them. There was not as much humor in this one, given the death toll, but it was a fun book, especially with the series of battles from the skies in the beginning to the Malfoy estate to the bank and then to Hogwarts. How they make this into two movies will be most interesting.
And the topic of best HP movie has to wait until 2011.