For this I would definitely have to say Allegiant by Veronica Roth.
If you read my previous post I Hate Veronica Roth then you at least know why I hate this book. On the other hand, the reason that I love this book is because everything starts to makes sense, and it puts a location on the parallel universe of the Divergent Series. It’s also nice to find out that Tris’ mom wasn’t only Dauntless but Divergent. Other than that feel free to read my earlier post to find out why I also hate this book with a passion.