Immigrants have changed American society in tremendous ways since 1865. It is impossible to say in any objective way which group has had the greatest impact. If we look at our society today, we might say that Hispanics have had the greatest impact. But that might simply be caused by the fact that they are the most recent immigrants. It may be that other groups that we hardly notice anymore have made the greatest impact.
The main way in which immigrants have changed our country is by helping to make it economically dynamic. Throughout much of our post-Civil War history, large numbers of immigrants have provided the labor that has helped the US to grow. Scandinavian and German immigrants farmed the High Plains. Irish immigrants and Chinese immigrants helped to build the transcontinental railroads. Immigrants of all ethnicities provided factory labor as America industrialized. Without this immigrant labor, our country would not have grown as fast as it did economically.
Immigrants have also changed our culture. Just to take one obvious example, various kinds of immigrants have brought their foods to our country and given us a very diverse diet. Immigrants have brought us some of our most “American” traditions, such as the Christmas tree.
It is almost impossible, though, to say which group has had the most impact. We might say Hispanics have had the greatest impact because the changes they are making are visible today. On the other hand, we might say it is the Germans who have had the greatest impact since people of German descent are, by some accounts, the largest single group in our country today. Though they have essentially disappeared as a separate ethnic group, we might have to conclude that they have had the greatest impact since there are more of them than of any other group. This must mean that they have affected our society in many ways.