Goblet of Fire is still my favorite. Half Blood Prince and Deathly Hallows are about tied for second.
Things I liked: Starting the book right off (basically) with a fairly important character dying. I kind of expected Moody to buy the farm at some point, but I wasn't expecting it that early. It was a very effective way of emphasizing the sense of impending doom that permeates the rest of the book.
Fred and George making jokes about George's severed ear. Nothing can make those guys stop being the Weasley twins.
The way things seemed frustratingly slow during the early stages of the Horcrux search. I'm in agreement with you on that bit, Ape.
Everything Neville, Hagrid and my still-favorite character, Luna, did. They were awesome throughout the book.
The fact that "good" people weren't the only ones to destroy Horcruxes... I thought it an interesting twist that Crabbe helped Harry while trying to kill him.
Dobby's death scene. While it was sad, I'd been getting the impression for a long time now that he kind of wanted to die for Harry, so I thought JKR did him justice.
Learning more about characters that haven't been mentioned since the first book, like Ollivander, Griphook and, yes, Grindelwald.
Snape's death/Pensieve scenes. I thought that was a nice twist that he was in love with Lily.
Molly opening up a can of whoop ass on Bellatrix.
Things I didn't like: Ron being a dick in the early going. You're right on that point as well, Ape... it very much reminded me of JKR's earlier mistakes in book 5.
Xenophilius Lovegood. He was a jerk.
Lupin and Tonks dying. They'd just had a baby, for fuck's sake.
Voldemort's death scene. It seemed the introduction of
Sectumsempra, which I thought was a nice, not Unforgivable way for Harry to kill Voldemort, was wasted. It was a total pussying out on JKR's part. In fact, I was disappointed with the lack of mangled Death Eater bodies in general... I've always felt the Order of the Phoenix weren't really taking the war as seriously as they should, and their continued lack of willingness to use deadly force makes me think I'm right.
I also thought that reintroducing Umbridge for only a single chapter and having nothing worse than a Stunning Spell happen to her was a wasted opportunity.