I would say Lunarian over Oni
just because most of the powerful / cool shit King does is attributed to his race, while most of the crazy shit Kaido does is attributed to him being a Dragon or himself just being nuts
I think there has been a bigger focus on Lunarians being broken, just look how nobody questioned how the Onis could've been extint, but Sanji was like "wtf? how did they kill all these Lunarians then?"
But we still need to see more from Onis, and i think we will, when we find out what is up with Onigahsima, the Skull, and all the shit about them pulling continents around.