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I hope you don't mind if I reblog for some opinions of why who's who is the first option.
As for those watch the series, you know that what their personalities and mission they wanted to be. And it doesn't always mean they want to be a king.
Based of how I described it, I think we all agree that Agrom and Tasaru are out of the list almost quickly. But let me describe why they can't be a great king from the start.
Agorm: While he's a good leader, in the first episode, you can see he's uninterested with listening to politics, fighting nor anything when it goes with dealing with the kingdom. His only goal wants to adventure the outside of his kingdom. And at the same episode, you can see that even the guards didn't trust him when he's report the emergency when Firespitter appears since he's used to make up stories until it's too late (Reminds me of a farmer wants attention to eventually fake alert the other farmers that his sheep got captured by wolves and eventually got uno reverse when he realized that no one would help him when his sheep is actually captured by wolves.)
Tasaru: Born to be a Lord, forced to be a champion fighter. He never meant to be a Lord, but was trained to be a fighter since youth. While he can fight to protect the kingdom, a lack of communication and connecting the others are the main reason why he can't be a King. He also lack of support thanks to his master and being the youngest also push him out to be one. And he doesn't even identify as "prince" makes it more of the push aside. (And if the three doesn't be in Forest Kingdom, I almost think Forest Kingdom would be endangered more and he might had depression.)
Now we have Deron, Notics and Piron. The reason Deron is on the list because at the start, he wasn't even a king and have to fight Piron in order to take the throne. But I still gonna explain based of my knowledge.
Notics: He can fight and he had some knowledge for his kingdom. While he's determined and fit to rule the kingdom once he's old enough, the only thing that's pull it away that he's wanted to impress his father. Good thing but as we all know, his father neglected it most of the time, understandable as a king. But given his father always at his ego, which that's how Notics gets his personality from, I personally think one day he would do the same thing as his father if Agrom never went to Air Kingdom. As wise person say, a child would take their parent(s)' footsteps.
Deron: Deron might be as intelligent as Piron, but unlike Piron, his ego for throne is reach to the top. Sure he gets support from his people and have knowledge to actually rule the kingdom. But if he managed to take a role as a king from the start, his kingdom might be controlled by Magor almost immediately since Deron is almost immediately to be manipulated as he wants power, which is showed in ep 16 by Magor using fire mind.
Piron: And we all agree that Piron is the best choice when it comes to the king. He might have more knowledge than Deron. He barely trust anyone almost immediately (ik like everyone else would do). His gentleness, his great thinking and such, he would be such a great king. However he's passive, as he doesn't fight his own brother just to become a king, even if it's traditions.
Question that pop my head in one sec of GNU community:
Well in terms of:
Trained to be (Ik Ik they're still around pre-teens to teens, but they should be teach at least young age)
Smart enough to rule the kingdom
Actually understanding some kingdom stuff (probably like politics and citizens' stuffs)
Citizens' trustfully of them rulling the kingdom
Actually have some defense, fighting, strategist, knowledge etc...
A/N: I'm not including Notics's Dad or (Nadar or something I forgot) since he's the current king or any current king in every kingdom
Edit: It's Nadar with a d, not a t. Plus if you see me also vote, no you didn't :) (It was an accident)