This book also contained, in my opinion, more actual magic in it. We finally see Harry using a variety of spells that extend beyond his previous inventory that really only consisted of wingardium leviosa, exspelliarmus, lumos and expecto patronum. we also see other wizards using more complicated charms for the tournament and are introduced to real dark magic (unforgivable curses, voldemort's rebirth etc.) In addition, we finally see crushes entering Harry's world as the trio are starting to grow up and develop romantic relationships.
Overall, this book was one of the more magical and mysterious in the series on top of being the set-up book for the second, darker half of the series.
I loved all the books, but if I had to choose a least favorite, it would have to be chamber of secrets. I don't know what, but something about that book made me feel very impatient to reach the end. Maybe the story was just a little too long in terms of the chamber of secrets mystery. I still loved the book all the same though!