Mostly, because she wants to be a restaurateur, first, and the marriage and ruling a country would fall to second place for her.

Also, Merida, because she has a lot--- and I mean a lot of growing up to do. And first, Merida needs to start being empathetic more often, or she will run a country into the ground.

Finally, Rapunzel isn't at all ready to rule. If watching Rapunzel's TV show episodes was any indication, Rapunzel doesn't want to be saddled with ruling Corona just yet. Maybe, like, when she's 25 or 30 something.

The best princess who would become a ruler would definitely be Cinderella. Then perhaps Ariel and Jasmine, who seem to thirst for exploring things that are different from what they themselves know, and they actually care what happens to the people living in their kingdoms.

I wouldn't say she's the worst, but I'm sure she doesn't even want to be queen as she's a career oriented person and would be focused to building her restaurant.

Merida actually has potential to be a good queen, if she's grown up and learn to be more respectful and dutiful. The way how she's negotiate and speak with the lords already shoen her potential.

She was isolated to a tower growing up for a long while and if she doesn't learn enough and grow up in time, she will be a very naive Queen. No thanks to the Gothel...

Even though I like Ariel, nothing indicates to me that she's fit to rule a country.

I picked her for the same reasons people usually say Elsa is a bad queen, except with Rapunzel she doesn't have the life long training and education needed to run a city-state. She has tons of issues she's never gotten therapy for that the movie kind of just brushes off, which likely would have huge effects on her ruling and decision making skills. She's got a bad tendency to put her wants first. It's great to play with your subjects and let them get to know you but that isn't what makes a competent ruler. There's also that dhe completely let's Eugene (a criminal) off the hook when we know she wouldn't have with Gothel (another criminal). She doesn't rule fairly.

So, unresolved psychological issues, makes super emotionally based decisions and focuses on her own desires, not consistent with applying the law. Sweet, but doesn't make for a good ruler. That's part of why Tangled bothers me so much. It tries to paint itself as a "realistic" tale but the Punz's issues and Flynn's crimes are both handwaved away.

Also, if Tiana were to be a queen I'm sure her restaurant would be out of the picture entirely so that shouldn't really be a factor.