Ability-wise I'd have to say Killua. He is a trained assasin who has the ability to resist poison/ torture. And he manipulated his nen into electricty which seems to be a rare and difficult task.
Raw Power-wise though, it'd have to be Gon. For his ability to beat the living s*** out of almost anybody. Adding to that he has a very strong Nen and a temper that increases his strength greatly.