Ranked from absolute worst to absolute best, not on how they would do as a king, but how I would imagine them doing based on their campaign promises alone. I'm not including Will because his campaign promises are simultaneously vague and up to the player's digression.

Jin- Will say whatever it takes to win. This is worst case scenario, he just wants the title and has "concepts of a plan".

Julian- No more meat. He doesn't really have anything more comprehensive than state-ordained veganism. No replacement for the lost food in cities that are already starving, and to be frank, most of the food in Metaphor comes from monsters that want to eat people, meaning the cities are going to have a much harder time holding back monsters if they can't kill them, and a harder time surviving if they can't eat them.

Glodell- Give up immediately and scede to Louis, but Glodell may or may not actually do it since he's kind of wishy-washy about the whole thing. Probably worst case scenario since

Gidaux- Give up immediately and scede to Forden. This means that a vote for him is a waste of your time.

Loveless- Alcohol for everyone, let's party. In shrewder hands, you could see this being a grab at monopolizing production and distribution of luxury goods. In Loveless's hands... I don't actually see this being anything more than drunken pandemonium for a few nights followed by a massive economic crash.

Catherina- Just kill the upper class and replace it with a new upper class. This went fantastically in Rwanda, so I know it's a good idea.

Roger- Slightly less chaotic than Loveless, but the concept is the same. Just have no government. This comes with the destruction of infrastructure, public healthcare, and public schooling, and basically leaves a gap for the toughest guy to take over. He's basically sponsoring Louis even if he doesn't mean to.

Goddard- Give power to the old, tax the young. As far as I can tell, this is the current system when the game starts. Goddard just has no other actual plans.

Forden- Maintain the status quo established by... Forden. This is the control group. I think anything below this is better, anything above is worse.

Milo- Grant all power to people based on how beautiful they are. This seems stupid, and it is, but it, in a roundabout way, does ensure a democratic system since beauty is a societal decision, and because some hallmarks of beauty are based more or less on health, it ensures there isn't an impending FDR situation where the leader withers until he's a sad little husk in the office.

Rudolph- Turn the nation into a military state. This sort of government does exist. North Korea is a horrible impoverished sad blob of land, but it is a nation that persists, so it is almost an option for a government. Rudolph has an advantage on Kim Jung Un, since he has combat experience even if he's not in the top 3 fighters in the royal tournament.

Louis- Get rid of the church and establish a more equal society while killing Humans. His speeches are very strong, even though that's 100% not what he actually plans to do and he lies through his teeth in every speech.

Edeni- Religious freedom. That's it. Implicitly he'd also include his nation in with the nation he took over, meaning he's casually offering complete religious freedom and the end to tensions in the area. Not bad campaign promises, honestly.

Lina- Industrialize the nation using her family business as a leadership model. Look- I'm not saying a nation has to go into mass production to be successful, but I am saying China's wealth has been entirely based on how much production and distribution it has been able to put out for the last 20 years or so. Lina might be cooking something pretty nice.