Canon Royale? That's certainly a question. To keep things brief, I'll exclude everyone who's just an ordinary human, but that still leaves a good few people. Haru, Edelgard, Falin, Aster, and Hoshiko all make the cut without really being able to compete with the heavy hitters. Suletta may or may not stand a chance, depending on whether there's a giant robot available. There's also Battler. The power levels of Umineko magic are kind of insane, so they're probably making it to at least third place. The Long Quiet, as a god of sorts, easily beats out anyone else in the system when it comes to power level, but I can't say they're a shoe-in for first place, because of... Subaru Natsuki. If Return By Death is functioning, it becomes effectively impossible for anybody else to claim victory without causing time to loop, essentially allowing her to win by default.