I am so down for King Edmure, first of his name, protector of the realm and advocate for the small folk. And the Blackfish as the Hand of the King...there might actually be some intelligent people on the throne.
Doran and Olenna are probably the best living candidates in Westeros. Both are wise, careful and experienced in ruling. But Olenna is irreverent and grumpy, while people just don't tend to respect Doran. Neither is really kingly, so maybe they're best as advisors to more dashing younger relatives.
I still think that Littlefinger would be a good king, if Littlefinger had an heir that he loved. Littlefinger is so very competent. And Littlefinger is doing a good job of managing the Vale so far - he's a lot less destructive when he's on top.
I also think that (show) Margaery + Tommen could be a good monarchy if they were able to get rid of Cersei and had five quiet years to get some experience under their belts.
Randall Tarly is a pretty loathsome person but he is a military leader in his prime, he doesn't have a lot of enemies, he's respected for his competence, and he upholds rule of law.