In the past, Half-Blood Prince is the one I found myself going back to most often.

Even though Order of the Phoenix is my favorite film, I wasn't a huge fan of the original book; it just seemed to be taking things a little too seriously to be enjoyable. I know it was meant to be centered around the ramifications of Voldemort's return, but Half-Blood Prince managed that better without taking such a steep dive in terms of mood and while still portraying all the normal goings-on at Hogwarts.

There are other things I like about it too, like how it explored the details of Tom Riddle's backstory, and that it did away with the frustrating element of Harry, Ron, and Hermione suspecting someone of doing something nefarious, only to be proven completely wrong at the end of the year. It was always difficult to stomach that plot thread in the previous books, and I was glad to see Year 6 take a subversive stab at it.