Tyrion would be good if he could keep it...which I'm not too sure on. He's a patricidal manipulator and his enemies would use that against him, no matter how good he'd be at it (being the right combination of smart, ruthless and decent).
Stannis would be a bit more stable. Blood right is irrelevent, but he'd produce a climate totally opposed to the backstabbing politics. He's not subtle enough to deal with that, he'd just smash it to pieces. Again, this would be a hard rule to actually sustain, although much easier than Tyrions. The primary problem would be his lack of an male heir and his crazy fire priestess. Mil would provide a few new options, but she'd promote a similar reaction to the Mad King with the whole "burn all who oppose me" thing. Shame. Davos would make a good Hand, I think.
Dany's destined for nothing good, I think. She's slowly but surely sinking into her family's traditional madness, made worse by the fact that she thinks she's all about justice. The dragons are more dangerous to the people than just about anything else and her control is slipping faster and faster. Wouldn't trust her with the Iron Throne, although there would probably be a brief period of decent rule if she gets control before her sanity breaks down further.
There aren't many other valid candidates, either being too weak, too crazy or just plane not around to pull it off. With that in mind, I hereby promote Qyburn as King of the Andals and the First Men, Protector of the Realm etc. because I think it would be funny watching him use unprecidented power to do exactly what he's currently doing, but on a bigger scale. Then Essos can invade and it becomes Mad Science vs. Magic (well, dragons) and everything explodes colorfully.