The grand conclusion. Leaving me a little wiped out from the charge through them all. (though I have concluded that, yes, Prisoner is my favorite still.)
Spoilers for the earlier ones.
So after the final appearance of the Dursleys, a big group sets out with seven Potters -- six people take Polyjuice to look like Harry -- and find they are chased by Death Eaters -- they had been betrayed. Mad-Eye does not make it. Mrs. Weasley puts Harry, Ron, and Hermione to work in an obvious effort to prevent their making plans for their quest, but they manage.
Despite the wedding's being the next day, they throw him a birthday party, during which Mrs. Weasley gives him the traditional wizardly coming of age present: a watch. This one belonged to a now dead brother. If you picked up the clues carefully, you will know he was a member of the Order of the Phoenix killed by Death Eaters with his brother Gideon, after fighting like heroes. You could write an entire book about the motif of gifts in Harry Potter, which is deployed frequently and to great effect, but I think this one of the most powerful usages.
And then it becomes very important that they managed, because they get to start in the middle of the wedding: a Patronus arrives to warn them that the Ministry has fallen, and the wedding will be attacked. Hermione brought their supplies, and they bolt. Off on the quest to destroy Horcruxes. When they don't know where any of them are.
This involves Polyjuice Potion and Arthur Weasley berating Harry because another wizard is worth ten of him, or rather, of the wizard whose shape he's got, the trio's discussing whether it matters if Voldemort possesses Dolore Umbridge, given that she's so evil already and Umbridge's demonstrating that they have a point, a grandmother's writing to her grandson that she's on the run for her life because of him and she's so proud of him, a ride on dragon back and hysterical laughter, memories from Snape[Spoiler (click to open)]I think Voldemort may have intended to let Snape have Lily, whether from contempt at his being a half-blood or in hopes it would satiate his desire; his usual tactic for people who got in his way was not to tell them to step aside., Fleur's asking Remus about his son in an attempt to cover an awkward silence, murderous persecution of the Muggleborn, a wondrous scene in which a silver doe leads Harry through a wintry forest, many occasions where Voldemort demonstrates that you don't want to work for him, and much more.