I used Romance of the Three Kingdoms and Spycraft as a guide... but I have to tell you, DnD does not do leadership well in the core rules.
First of all, IMO there are two kinds of important people, those who deserve their position, and those who inherited it.
If someone just overthrew the king and founded a new dynasty, he's probably a high-level character with a high Int score. There is no good PC class in core DnD to cover it, but I suggest the Noble or Marshall, or similar classes, from some DnD supplements. Or make your own. (I can't stand how the Marshall's tactical abilities are FX!) I'm also tired of leaders being high-level mages or barbarians with no tactical or social skills (eg look at King Obould's stats in the FRCS... even before he became a "Chosen of Gruumsh" he was a very different character in the novels than he was in the FRCS).
I'm also tired of the highest-ranking warrior in a regiment simply being the best fighter. Yeah, he kicks butt, but that says nothing about his leadership abilities.
If the leader worked his way up the ladder to become town mayor or head of the assassin's guild, again, he's probably high level.
If he inherited his position, he's probably lower level, and doesn't strictly need high Int. However, some rulers insist their offspring work to get to their position, so their offspring will probably have some levels on them. In the Eberron camapgin setting, you'll notice lots of kings who are Aristrocrat 3/Fighter 1 and things like that... guess how that guy got his position? (Most of the Eberron kings range in level from 4 to 13.) While monarchs-in-training are supposed to get actual training, it doesn't always work out that way. This is especially true if the previous ruler thinks he will live long, or if he only trained his oldest son, but said son dies in an accident, and now he has to train that wild 17 year old fop how to be a ruler ... etc.
Regardless of how good the person on top is, he's going to have henchmen and advisors. The henchmen are there because the leader can't be everywhere at once, and are frequently better than him in one or two aspects (eg his first warleader might be a fighter/marshall with a better BAB and troop-leading ability than the leader, his champion is a high-level fighter with a better BAB, his bodyguard is a dwarven defender with ... you guessed it ... a higher BAB, his chief warmage, a Sor 10/Marshall 3, casts spells but good, and his two court mages are also potent spellcasters but they don't have tactical abilities or good Con scores, his spymaster has a really high Diplomacy score in order to subvert people and his own minions are mainly mid-level rogues, etc).
His advisors are probably lower level but maxed out some crucial skill (eg Sense Motive) that the leader does not have.