Meteor City. The reasons are really complex and personal for me - I would need a book to go into detail. I grew up in the ghetto, in the city of Flint, MI, a city much like Meteor City. From its status as an abandoned place, a junkyard and dumping ground for other people's waste, a city with very loose, if any, structure or governing, and the close ties of the people living there to the city, despite, or because of, its harshness. From the moment the idea of the place was first introduced through Canary (a person of color no less), and then to have it be expanded upon through the Genei Ryodan, I was just really invested in it simply due to the fact that a manga author was actually making an attempt to depict a ghetto with intellectual respect and emotional consideration.
As well as the fact that it is has been the womb for some of the strongest humans in the HxH world.