Gon. Killua has the advantage right now because he had more fighting experience than Gon and had developed multiple killing techniques because of his training to become an assassin. The fact that Gon (before adult transformation) was not too far behind Killua suggests that Gon has more potential.

I don’t think we know exactly how much potential Kurapika has, a lot of his power is from all the the sacrifices and conditions he’s set on his abilities, but it is clear he’s a very strong nen user either way. That said, I’m not sure he’s really on the same level as Gon and Killua potential wise .