While Prisoner of Azkaban is the best individual novel in the series, Half Blood Prince combines a great individual novel with a perfect book for its place as the penultimate novel in the series. This novel is the most focused on Voldemort but in the best way.
And most importantly, the titular character (spoiler warning) is also Rowling’s best literary creation. Severus Snape is the best part of the Harry Potter series. He’s the complicated antagonist that brings out the best in this series. I finished this book and knew fans would be debating Snape until the final novel was released. He’s that important to the narrative. For as long as Half-Blood Prince is, it doesn’t feel like it meanders around nearly as much as the prior 2 novels (and Deathly Hallows) because every scene really matters to the overall narrative of the series. This novel is Rowling at her best.