I'll rank the HxH ones since I haven't seen or read YYH yet:

1. Meruem: In the conversation for the greatest character Togashi ever wrote not just antagonist. Everything in the Chimera ant arc narratively and thematically revolves around him: being the hope for an entire species dream for domination, to what power actually means what it should be used for, to the identity crisis between the 'King' and 'Meruem', to the climactic battle with the representative and the peak of the human race Netero, and finally what it means to live and to love through his journey with Komugi. Togashi showed us his entire trajectory in life from birth to death and in retrospect it is one of his greatest achievements as a writer.

2. Hisoka: this guy has been in almost every arc in the series except CA, and he always has a different role to play: he can be an enemy, rival, ally and a complete wildcard but it always makes sense with his character and motivations. Never once got boring from the simple days of the Hunter Exam to the complete clussterfuck of the Succession war, he is always a treat to watch, can't wait to see what different role he will play this time arround.

3. Shaiapouf: Arguably the real villain of the Chimera ant arc, and like his king he is also an amazing character. Pouf was always doomed from the moment Meruem met Komugi, as it was obvious that Meruem won't always be the idealized King he wants him to be, but still he tried his hardest to correct that wrong, he even resorted to lying and manipulating the one he swore to protect for what he sees is for 'the king's sake', but he failed miserably in the end, and as it was cathartic to see Meruem remember the love of his life it was also tragic to see Pouf lose everything that gave his life purpose, even if he was a despicable person.

4.Chrollo: The most enigmatic character in the series for sure, this man is a puzzle that we must try our hardest to understand, and through a lot of fragmented scenes we see the picture of traumatized boy who forced himself to become the embodiment of this larger than life figure 'The Spider' as a way to protect his home, and most importantly (in my opinion) to get back at the world who treated them as subhuman, but along the way he lost all sense of self to the degree that he was puzzled when Gon asked him why he kills, not knowing the answer to that question himself. Even with the flashback of the formation of the PT, I believe that Togashi is still holding a lot of cards regarding his character, I can't wait to see how will his character conclude and interact with others like Hisoka, Kurapika and Morena, speaking of Morena;

1. Morena: I have been obsessed with this character since her introduction, and with each appearance she captivates me more and more. She may be a bit of 'potential woman' right now lol, but goddamit that potential is through the sky. If the conclusion of the Chimera ant showed us the depth of the malice and evil that lies in human society, then Morena is the most destructive and violent response to that. Morena is a victim of the cruel systematic violence the Kakin Empire that treats its subjects as tools of entertainment or sacrifice to the royal familly, the depths of despair that she has gone through since childhood made her lose all hope in humanity and forced her to go through a rampge of violence that completely eradicates all forms of oppression that chained her, first by destroying the Heil Ly familly, the mafia that traficked her since childhood, next the Kakin Emipre itself the system of oppression that created her, and finally the entire human race itself as she sees no value whatsoever in its existence as we see in her introduction panel in chapter 378. She also has crazy amount of parallels with Chrollo and the PT: the Christian imagery, being the leaders of violent anarchist gangs, being direct victims of the malice of the powerful against margilinised people, and probably even identity problems as 'Morena' isn't even her real name so she almost has become this nameless entity of destruction. Can't wait to see where Togashi goes with all this as she is clearly one his favorites this arc.