I have been wondering about how our perceptions of leadership, and things people today desire in their leaders, may differ from those in the medieval period. So, what would be seen as 'ideal' traits for a medieval monarch/king?
For example, would they want him to be a good fighter and a good military leader? What about a diplomatic and cunning leader? Would diplomacy, or military skill, be respected more in a king?
I can give an answer on medieval England here, but I'd be really interested to hear how the "ideal ruler" was constructed outside Christian Europe.
Sources on abstract ideas like what makes a ruler "one of the good ones" in the middle ages come mostly from chroniclers and literature. In some cases, like the continuations of the Brut chronicle or Adam Usk's chronicle, the authors are looking back on recent history and making judgments on what they see. In other cases, especially in Europe, you have a whole tradition of these political treatises that descend from the Latin De Regimine Principum that literally tell you what a good ruler is like. But the trick here is to think about what exactly the sources are doing when they write these things--you need to think about that everywhere, but the middle ages have some special rules. These authors came from a scholarly elite; being able to read and write well were very hard-won and very expensive skills held by people with the right mixture of wealth and religious connections. On top of that, manuscripts were crazy expensive and labor-intensive to make. That tells us two things: anything that gets written was very deliberate, and it was commissioned by someone with enough money to decide it was worth the money. All that to say, when you're looking at Thomas Hoccleve's Regiment of Princes, you aren't looking at an educated man's crack at judging his time, you're looking at an educated man's labor funded by and invested for very rich and powerful people. That's not true of all medieval writing, but you can't get away from that in political writing like this.
The Lancastrian usurpation gives us a pretty good case study of how a medieval regime could construct its public identity. In 1398, King Richard II of England confiscates the estates of John of Gaunt, and John's exiled son returns in a fury, gets support, and kicks Richard off the throne by 1399 and becomes King Henry IV from the house of Lancaster. But Henry and his supporters knew they couldn't just swoop in and say "I'm king because I'm stronger, so there," so they take the obvious road: they force Richard to say that he's voluntarily giving up the crown because he's been such an awful king.
No, really. Lancaster spread the idea in parchment and ink that Richard had these glaring character flaws that "everyone knows about anyway," and they even drew on the recently translated De Regimine Principum to point how how exactly he was bad. That text in particular talks about how, for instance, good rulers always act in the interests of the common good, and it draws on the Biblical principles about putting away childishness to outline how a ruler needs to be mature and even-tempered in everything he does. Just like that, Henry constructed a public image of a childish and selfish (thus tyrannical) Richard. Those virtues in particular were good for Henry to cultivate in his situation; his supporters turned on Richard partly because Richard seized Henry's inheritance and made them all thing "well shoot, that could happen to us, too!" Circumstance created a new "common good," and Henry played that up after the usurpation to make it look very legitimate and right that he take the throne.
In short, the nobility could renegotiate what made a good ruler. Kind of like prophecies, the "what makes a good king" literature advertised fairly general virtues that any good Christian ought to be following: patience, chastity, mercy, magnanimity, etc., but it was easy (by design) for rulers to pick up virtues selectively that worked for them. In a much broader sense, for most average medieval commoners, what specific qualities the ruler has is kind of a moot point; it's your ruler until said ruler kicks the bucket. As far as whether it was better for them to be more diplomatic or militaristic, again, it kind of depends on the circumstances that produce the sources around the king. Richard II, for example, wasn't a very warlike king. He wasn't keen on the war with France, and you can see in images of him that he preferred to style himself holding a scepter and orb--a judge--rather than a military leader. Incidentally, many of those who helped kick him off the throne were warhawks. Edward III, on the other hand, is usually styled holding a sword up with crowns impaled on it, sometimes wearing armor. A monarch's "virtue" was something to be spun not unlike modern PR teams handle our own leaders' "virtue."
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