Alright, you want my opinion? It's Janga.
The tension between him and Guntz is so thick, you could cut it with a Wind Ring Sword. Plus, he had tonsof character, and, well...I don't personally know anyone else in the Klonoa franchise that's an outwards and flat-out drunk. Alchohol? In a Klonoa game? That kinda speaks for itself.
So, he's intimidating, a dangerous threat, and unlike the other Klonoa villains like Ghadius, King of Sorrow, or Jillius, whom you'd no doubt feel bad for beating into the ground (for various reasons), Janga is someone you'll absolutely LOVE to HATE, and beating him as a result makes it SO MUCH MORE GRATIFYING. And you'll still hate him, even after beating him...
Garlen's a close second, of course. But he's not TEH BEST villain IMHO, so I won't be talking further about him.