The best assault rifle in Call of Duty: WW2 has got to be a versatile weapon that is powerful at medium range and should be able to compete with submachine guns at close range. The Bar rifle is a strong candidate as it has a low time to kill, has good range and is easy to use.
However, at close range the low amount of rounds per magazine makes it difficult to deal with many enemies in quick succession. The STG44 rifle is just as good as the bar at medium range but has a lot more rounds per magazine and is even easier to use at close range than the Bar. It is a good all around weapon that can be used to lead the push on objectives. Since the objective push is the main role of rifles, and the STG44 is very good at this grunt work, the STG44 is the best rifle in Call of Duty WW2.