I agree with Dae, I say Joka. It isn't a perfect comparison but he's got a similar connection to Klonoa as Janga does to Guntz, in a way. Not as directly, but, idk. Joka has appeared so many times and had probably the most personal rivalry with Klonoa in the first game.
It's kinda weird imagining just one villain as Klonoa's arch nemesis, though. I don't think Klonoa's the type to have an arch nemesis, he just fights whatever evil comes around. Joka on the other hand I can absolutely see declaring himself Klonoa's arch rival, haha