After successful military campaigns, I often end up earning the ire of my vassals for "too many duchies". To resolve this, I go to duchies I don't want, pick one of the counts in that duchy, and make them duke. If duchy does not exist, I create a duchy, and then do this.
However, I am actually not sure how to pick the best duke. I generally pick the count in the duchy's territory who likes me the most to minimize revolt risk. However while picking a -100 vassal is obviously undesirable, in practice the boost from being made Duke ensures that whoever I pick will have a pretty positive view of me anyway - so the revolt risk is low either way. Also, once the duke dies, there is little assurance his successors will like me as well.
Since my main factor, relations, fails to discriminate much between candidates, I am forced to essentially pick at random. I feel like I could be doing a better job.
Is there some kind of strategy to selecting a new duke? What are important factors to consider?
For instance, I can see that when given the choice between an ambitious and non-ambitious duke, the latter is preferable. But what if there are multiple candidates none of whom is ambitious?