The best American Infantry weapon had to be the M1 Garand. Even General Patton credited the American semi-automatic rifle as one reason we won the war. While other nations were equipped primarily with WWI vintage bolt action rifles that held no more than 5-6 rounds, the Garand held 10 rounds and was semi automatic. This increased the fire power of a 12 man squad dramatically over that of a similar squad armed with only bolt action rifles. The Germans also introduced a semi automatic rifle but never in large numbers.
The best German Infantry weapon were actually two, the MG42. this light air cooled machine gun replaced the MG34. It was easier to build and maintain and barrels could be rapidly changed, something important to sustaining a high rate of fire. The German infantry squad tactics were different than the Allies. The squad consisted of two light machine gun fire teams. The six rifle men in the squad were there soley to support the machine guns. The US forces had a different outlook and light machine guns were used in support of infantry. The American counterpart of the MG42 in the squad was taken by the BAR. It being a magazine fed (20 rounds) autorifle, it could not produce the sustained firepower that the two MG34s could. So on a squad to squad basis, the German squad could produce a lot more fire down range than the US squad could.
It has taken 65 years, but the US Army now emulates the German Wehrmacht. It now has squads based around two fire teams which have M249 SAWs or Squad automatic Weapons or light machine guns. We have learned that a smaller 9 man squad based around two light machine guns can produce more firepower with fewer men and are easier to handle and maneuver than the larger 12 man squad. The Germans learned this a long time ago.
To show you how good the MG34 was, it is still in use today by the German Army and is the basis for almost every light machine gun fielded since...