First of all, we must note that every immigrant group has assimilated “to one degree or another” since that could cover everything from complete assimilation to almost zero assimilation. Thus, you could answer this by simply listing any two immigrant groups that have ever come to the United States.
Perhaps the group that has assimilated most comprehensively into American society is German immigrants. German immigrants have been coming in some numbers ever since colonial times. There was a major spike in German immigration in the middle decades of the 19th century. Germans were not well assimilated by the time of World War I. This helped to cause a backlash against them that coincided with the war. However, in modern times, Germans are so assimilated into American life that we would not even identify them as an immigrant group.
The groups that have assimilated the least are the groups that have been here for the shortest amount of time. One such group is refugees from various African countries. There are many immigrants in the US from parts of Africa like Rwanda. Since they have not been in the United States for long, they have not assimilated to a great degree.