It would be nice if there would be a guide with the description of every technology that every immigrant has.
Now I think Polish are the most suitable for Argentines but make sure you also selected Catholicism, because Polish also benefit Catholic technologies.
Japanese and German immigrants are great as well, so every civ that has them available should use them. Germans can ship 2 factories so they can boom a lot of resources and they also make villagers tougher and cheaper, plus cheaper Trading Posts and native units. Japanese can unlock all mining technologies so you can get coin faster and they can give you +100% HP to your explorers and Forts.
in fact I always choose Catholicism when possible, since it is the religion that offers the maximum of technologies to increase the gather rates of the villagers,besides, I am a Catholic, so it is very cool to be able to choose my religion in the game.
Who would the best immigrants be for Colombia and why?
As I said in an earlier post the Boom is the best way to use Latinos, this way, at least in my opinion. Starting from this assumption I choose the ones for the Colombians the Irish immigrants. Because? Because they offer an interesting opportunity to hire mercenaries in a sallon, they can expand their limit of warships (+1 monitor always helps) and finally has a great skirmisher (cheaper than the Independentista, occupy less population space, has reach greater, well the Independist one only gains in the power of fire) and a strong cavalry that is better that the Colombian cavalry, since this civ is oriented to have a strong infantry