I'm more interested in which passport adds the most most visa free travel to a given passport. Maybe Russia would be best given a US passport, since Canada/USA/

I'm more interested in which passport adds the most most visa free travel to a given passport. Maybe Russia would be best given a US passport, since Canada/USA/Germany are sure to have a great deal of overlap. You can take the set difference of the passport you have and all of the candidate passports. Whichever candidate yields the set with the highest cardinality is the passport to pursue.