I'm not sure if this book or Sea of Monsters is my favorite in the series. They are both fabulous.
Spoilers for the previous books will follow.
This includes the continuation of poor Nico's story, and although he is certainly not the central character, we get to see a decent amount of what he has been up to in the time between his previous disappearance and now. I admit to having a soft spot for Nico, who ended up growing up a little too fast and has a tough burden to carry as the son of Hades. He causes some problems, but they are far fewer than they could be, and he does not strictly mean to cause all of them.
Lucas, of course, makes further appearances and remains a huge pain in everyone (expect possibly Annabeth?)'s butt. I understand they have a history, but she cuts him a bit too much slack, I feel. He is, as always, full of plans and causes Percy no little misery throughout. Kronos is an ominous presence, but still at least mostly in the background at this point, leaving Lucas feeling like the real villain of the piece.
The Labyrinth itself is fascinating. It's very much up Annabeth's alley, and I'm glad to see her get to take the lead a bit. One of my favorite things about this series is how expansive the mythology is--we get to see touches of all the tales and characters that populate old Greek stories, and it is amazing to see the tiny twists on them that are made and to see them populated by "real" people with problems and genuine feelings, not just "heroes" and "villians" and "gods".
Percy, as always, causes loads of trouble, and remains remarkably blind to a certain someone's interest. Rachel Dare causes a bit of trouble on this front, but it is easy to remain hopeful if your shipping interests lie elsewhere.
Annabeth gets to take center-stage a bit, as this quest is hers, and watching her come into her own is an amazing thing. She is brilliant and charming and just so much fun I want to be best friends with her.
All of the characters have grown so much throughout this series. This is the book that probably highlights that best, given how action-filled and character-driven it is. There's less humor, but we're going into the big epic battle that we know is coming, so that feels quite appropriate.
This entire series is amazing and everyone ought to read it. I am going to continue harping on this point until it actually happens.