Ok, this one is my personal favorite. There are so many developments in this book. We get to know Thalia, we meet characters like Nico, Blackjack, Bianca, and the Hunters, and the relationship between Percy and Annabeth develops a lot. This is also the turning point of the series where things get a lot darker. In this book, we have the first death of a major character, and it's done very well. The characters react accordingly, there's fallout after the fact, and Percy in particular has a lot of survivor's guilt. It's done in a way that keeps it appropriate for the target age group while still imparting the gravity of the situation. We also get to see Percy in a more authoritative position. Up until this point, he's been the one receiving orders and being cared for, but his interactions with Nico in particular put him in a position of authority that he's going to be expected to fill more and more.