Hisoka for sure, he is pretty much an unique character (you wouldn't find another charcater like him anywhere outside of HxH), I think I appreciate about him his madness yet he is not crazy at all, well, he is crazy like hell in a way but he always thinks what is the best plan of action including his madness in the equation of course and Togashi mades this messed up character work perfectly making him a villain or an helper without that seeming odd at all. If there is something I didn't 100% like it was his ressurection but on the other hand it was obvious Togashi could not lose Hisoka like that, it would be like shooting at his own foot in the middle of a 100 m race.
So he has to come back in some way and, well, that was one (enojyed it nonetheless yet I was not 100% fine with the faked death, see chapter 900 of One piece, even if of course liek already said, count he is Hisoka and count there was the experiment he did while dying he had to come back).
Then probably Gon. Gon derails quite a bit for your usual shounen MC, while Killua follows more the usual troupe of the shady shoulder character (Sasuke was "inspired" by him and Kurapika) but indeed maybe I'm talking like this since we have Hiei and Sasuke who already did something similair (one is from the same author while the other was heavily inspired by him but become way more famous).
Then... Gin? It is really difficult to choose with so many awesomelly well done characters to choose from. What I like about Ging is his character for sure and the 4D chess match he played with Pariston while the other zodiacs were clueless was awesome.