WRITTEN BY RIYA DUBEY
Although these two close friends will never fight each other in a real battle. In a hypothetical all-out fight, Killua would most definitely win against Gon.
Killua had been killing and fighting for most of his life. This means he has a much heater fight experience than Gon, and unlike Gon, he isn't afraid to use unethical methods to win.
Whilst Gon indeed won against Neferpitou because of his nen potential. Gon made a deal that granted him great power in exchange for his life force.
Moreover, he was able to make that deal due to his negative feelings, but that wouldn't happen with Killua.
Gon has more firepower due to his Jajanken. Killua on the other hand has his godspeed. As mentioned, Killua isn't afraid to kill and would most likely fight with the intention of eliminating Gon.
This is something that Gon wouldn't do. Both of them are aware of each other's weaknesses, but Killua has an upper hand in this.
Whilst Gon is unaware of several of his weaknesses, Killua is very self-aware, and because of this, he would know which weaknesses Gon might exploit.
Therefore, even though both of them are very strong, Killua would most definitely win in a fight between the two.