South America? I don't think there are many immigrants in Europe from South America other than to Portugal and Spain due to their historical ties.

The easiest immigrants to integrate in Europe apart from other Westerners seems to be people from the far East from countries such as Vietnam, Thailand, China and Philippines. Tamils also have very good track record in my native Norway. They tend to have very good school results for instance.

But, if we were to accept people form the poorest regions, then I think the Christian part of Africa probably works fine. A big problem with integration of muslims is that they don't marry locals.

But I am not sure how you prioritize groups from particular areas without creating a racist system. In general most European countries don't have open immigration. People come primarily due to wars as refugees or due to family reunion or as skilled labour.