Man, this is the kind of debate that can start a brawl in the tavern at the magic knights' headquarters. On paper, obviously it's the Wizard King. The title literally says it all, the supreme commander of the Clover Kingdom's military might, answering only to the King. Julius Novachrono embodied that power perfectly for a while, his time magic was absolutely busted.
But the question is about 'penguasa', ruler, right? And that's where it gets messy. Because the actual, sitting king, Augustus Kira Clover XIII? Dude's a joke. A vain, cowardly figurehead obsessed with his own popularity. Real power flows to the Magic Parliament, the nobles scheming in the shadows, and the captains of the nine squads who basically run their own mini-fiefdoms. The kingdom is less a united realm and more a loose collection of powerful guilds under a shaky banner.
So the most powerful ruler isn't a person, it's the system itself—the entrenched noble class and the military might of the Magic Knights. Anyone sitting on the throne is just a figurehead unless they have the sheer, overwhelming personal power to back it up, like Julius did. Post-timeskip, maybe that changes, but the structure itself is the true ruler.