Harry Potter and the Deathly Hallows
I'm gonna stay spoiler free here, but wow.
At first I have to admit I was dissappointed, but once I saw where she was going, it all made sense. This book is about nothing less than total war between wizards. And it starts quick and it doesn't stop all the way through.
What was a little scary is that I was mostly right in my own predictions. Either I have a hidden genuis or JKR is really predictable.