Hunter Exam Arc: The "contest" arc, in which several characters need to compete with each other in order to win a title. This arc examines what would actually happen in a contest filled with superhumans- the normal humans die in brutal ways, the empowered individuals run rampant and murder dozens, and Gon barely survives only because the only two people that know Nen are Hisoka and Illumi, who likes Gon enough to keep him alive for Hisoka and barely processes Gon's presence for Illumi.
Phantom Troupe Arc: The "villain squad" arc, in which the heroes face off against a group of evil villains. This arc examines what would happen if you took superpowered humans and put them in the real world- the power gap between the troupe and the mafia goons is massive, and only Kurapika, who has limited his arsenal to specifically only fight the troupe can stand a chance. Even still, Kurapika can only do so little, and most of the troupe is still alive.
Chimera Ant Arc: The "enemy army" arc, in which a large enemy force accosts the heroes, with a powerful leader. This arc in particular examines the idea of having an enemy that is significantly stronger than you- from the enemy's perspective. What is it like being devastatingly stronger than everyone else? How would something that is on another level from everything process the worth of other creatures? What would something that was meteorically stronger than everything else think when it realizes that other creatures can think just like it can? What would it constitute as strength? The Chimera Ant King is a rather somber answer to all of these questions. Even more deconstructive is that the humans actually cheat to win- upon learning that he will die and realizing no one else can stop the king, Netero detonates a nuclear bomb in his chest than eventually kills the King from radiation poisoning, rather than a straight up fight.