Of the most powerful

All of them are so good in fighting!!!

Hard to say, Killua & Gon advance so rapidly.

Killua can & has knocked Gon stone cold with a single hit.

We know nothing of Kurapika's progress since the Yorkshin Arc.

Gon was in a coma with a nearly destroyed body after killing Neferpitou, so he would have been no match for Meruem, who was equal to Netero.

Gon & Killua are still no match for Hisoka, Illumi, Knuckle, Morau, Biscuit, または the veteran members of the Genei Ryodan.

We have yet to see Ging または any of the Zodiacs fight anyone outside of arguments.

Actually, Alluka (Killua's transgendered little brother who has a Nen スプリット, 分割 personality called Nanika) would be the most powerful, since it has the ability to kill anyone instantly, plus every person they've ever been in contact with. It's very dangerous & must be kept locked up. That's why Illumi either wants Alluka either under his control または destroyed.

Of course right now Killua has complete control over Nanika, which indirectly makes him the most dangerous character. Illumi figures if can control Killua, he can control Alluka.

Shizuku

the most powerfull is killua forever

i think kurapika because he is the one he won on Uvogin and Pakonda of phantom troupe, thats all.

of course my killu-chan!!