Yes, Killua has the most potential in the history of his family. This includes Maha [Zeno's Grampa] - it's been stated that the only person to ever survive against Maha in battle is Netero and keep in mind this is likely a Prime Netero. The statement from Zeno can be interpreted that Maha was at least on par with Netero.
His current level is unknown as his best feats are against the Royal Guards. The problem is that while he was unable to significantly damage them, would that be the same for non-Royal guards? We just don't know how well Godspeed Kilua would fare in a fight against non-ant opponents because the guards have OP durability.
I like him too.
Hopefully in the current arc he can really step up and show what he's learned in the medical field. Perhaps there's a scenario where some major characters are severely injured and the only one around to help them is him.
You're right. Its hard to tell where his skills are right now. Yupi was the one he used the attack on right? It seems like an attack that most people would really feel but those opponents were so overpowered compared to them.
And this arc probably really is where we will see more stuff for Leorio since he gets to show up again. I'm glad he got development at least in becoming a zodiac. I'd like to see more of his nen powers as well. He was the slowest to develop them but it looks like by now he has a lot more than he did in the past.