This is a difficult question to answer as there are several great plot twists in the Harry Potter books. However, one of the best and most unexpected plot twists in the series is found in the book Prisoner of Azkaban. When Sirius Black turns out to be the true killer of James and Lily Potter, it is a shocking revelation that completely changes the course of the story. It is this surprising development that makes Prisoner of Azkaban one of the most compelling novels in the Harry Potter series.