Well, I' say that if its the type of ideal feudal idea of government, where the powerful set themselves up as protectors of the commoners, who then support the rulers financially so that the rulers can devote themselves full time to defending the masses, then high level characters would e the most effective in this position. The archetypal warrior-king.
If its a more modern idea of ruler as adinistrator, someone who can coordinate everything and make things run smoothly, then its more about Charisma and ranks in Profession:Governing.
The Aristocrat class fills both of these role pretty well, I think. The right set of class skills, good skill points, and decent combat ability to fight when necessary.
I think the ideal governing body in a D&D society would be the classic aventuring party, a group capable of defending the commoners when needed, with the combined expertise in various areas to cover all bases. An Aristocrat or charismatic Fighter as the "face" of the group, to deliver proclamations and hold court, a Wizard as the advisor/administrator of the operation, a Cleric to serve as the divine intermediary (needed in a world where the gods are directly involved in mortal affairs) and a Rogue as an intelligence cooridnator, and a Bard as diplomat.
It would probably become dynastic, as the group would groom its descendants in thier roles, or select thier successors, and, as has been stated, equip them and get them levels so that they will have enough personal power to take the reigns of gaurdianship when it is thier time to assume authority.
So basically, I think the classical feudalistic D&D model already in place is the most logical outcome of D&D reality in regards to government. The King with his Court Wizard and High Priest advisors and the head of the Thieves Guild as advisors, all running the show, and the Royal Heral maintaining alliances, all of them passing thier titles down through thier families. Thier woul be knights and such there to fight the mundane threats, but when the rampaging dragon comes terrorizing the countryside, the King and the Royal Court come out of the castle to do battle with the beast and fulfull thier obligation.