David: Deathly Hallows
I have just finished reading the final Harry Potter novel. I know what you're thinking ("he can't have read it, it isn't out for two months!"). Just know that I have connections, and those connections are legit.
So, here's the big ending!
Snape isn't really a bad guy at all, and he didn't really kill Dumbledore. In fact, Snape IS Dumbledore! What's more Dumbledore is a decendant of Salazar Slytherin.
The Dumbledore that Harry knew is really Harry's father, James Potter, who was really an alias of Sirius Black. This explains Sirius's overwhelming attachment to Harry, since he is actually his father.
What does all this mean? It's quite obvious isn't it? Voldemort is Harry's future self projected into the past and given a new name. In order to restore balance to the wizarding world, which seems to get more serious about itself the longer Harry goes to school, both versions of Harry must die. So Ron and Hermoine have to kill both of him. Snape/Dumbledore/James/Sirius watches this with glee, since his overarching plan throughout the whole series has been to place a large bet on a Quidditch match that can't be won without Harry. Slytherin wins this match.
Ravenclaw gets the house cup.