I can think of so many reasons why son of neptune is a top 5 pjo book but a big one is just that the stakes are so fucking high for the entire trio. ALL of them have good reason to believe they won't survive the quest. percy just lost his achilles curse and ella's prophecy foretells that a son of neptune will drown. iris personally told frank that he'll die holding his firewood and then the quest demands that he burns it in order to melt thanatos' ice chains (+he's worried that thanatos will take his life regardless because of his firewood). hazel isn't even supposed to be alive in the first place and the main conceit of the quest asks her to fight to free the god whose very purpose is to kill her. son of neptune is a story about death and the terror of its inevitability. the gang was in a state of highly personalized abject mortal peril for most of that book and there is a very feasible alternate universe in which the quest to alaska "succeeds" and not a single one of them makes it back alive
