The United States for sure any time after Pearl Harbor in the Pacific,or maybe also Normandy in Europe.Before America(and I guess the rest of the "allies" such as FranceWhich were the most effective soldiers in WW2,the western allies,the soviets,the japanese or the Germans?
Overall,I would say the worst by a long shot are the Soviets pre-1942,they got thieir ass handed to them by,off all people,Finland.They invaded Finland to try to make it part of the Soviet Union,than the Finlanders fought back,literally often just skiing around the battlefield because they didnt have many vehicles.Soviets outnumbered them 3:1 and yet got killed on a ratio of 5:1.Eventually they ended up having to make peace with teh country and only got a little land.