I generally pick the count in the duchy's territory who likes me the most to minimize revolt risk. However, I am actually not sure how to pick the best duke. If duchy does not exist, I create a duchy, and then do this. To resolve this, I go to duchies I don't want, pick one of the counts in that duchy, and make them duke. After successful military campaigns, I often end up earning the ire of my vassals for 'too many duchies'.
