Save Harry Potter and the Goblet of Fire (Book 4)

The plot here is the weakest of the entire series. The baseline "life at Hogwarts" stuff is still fun, but the rest is a bit forced. As the central book of the series, though, it can almost be taken as a metaphor for the series itself:

The 3 tasks of the Triwizard tournament are effectively a recapitulation of the first 3 books. However, in those books Dumbledore was the one pulling the strings and making sure everything worked out. Here Voldemort is doing the string-pulling, though we don't find that out until the final pages. That reveal is when the world of the series changes from a place where a happy safe ending is guaranteed to something more dangerous and real. It is also a bit of foreshadowing towards book 7 when we understand that even Dumbledore doesn't necessarily have a happy ending in mind for Harry. Get more detail about Harry Potter and the Goblet of Fire (Book 4).