To truly understand the greatness of the following scene I will now describe the previous bit of plot. Through out the third book it is Annabeths task to follow the mark of Athena and to reclaim the lost statue of her mother that was taken by the romans long ago and is the main reason for the civil war about to break out between Camp Half Blood and Camp Jupiter in Order to avoid this they are trying to reclaim this statue. However on the journey to get there Annabeth is forced to fight the mortal enemy of all the Children of Athena Arachnia the Mother of all spiders after entering her lair they talk a lot and eventually manages to trap her in a chineese finger trap. In her rage the immortal spider breaks everything around herself and begins destroying the cave. Due to this a crack forms both in the ceiling and the floor.
So now that you know the context you need to be made aware that I am a huge sucker for romance and there simply is nothing better than seeing the characters you have been following for nine books by now and the relationship that has been building up for literal years pay off in such a manner. We know that Percy willingly jumps into Tartarus after having already felt the pure horror of it once before in the very when he was only 12 and having seen what it did to Nico who regularly spends time in other parts of the underworld as a son of Hades. However, despite all of that he still jumps to try and safe Annabeth and even after knowing they can not get back up to their friends without having to venture through literal hell he stays calm and instructs the others to wait on the other side. In conclusion I am just a sucker for romance and that is why this scene is my favourite not to mention that this leads to the best journey of my two favourite characters.