Harry Potter and the Order of the Phoenix


Returning for his fifth year at Hogwarts, Harry is stunned to find that his warnings about the return of Lord Voldemort have been ignored. Left with no choice, Harry takes matters into his own hands, training a small group of motivated students to defend themselves against the Dark Arts.
I like the Dumbledore's army aspect of this and don't know why this wasn't carried into the next. Most hate-able antagonist: Umbridge.