I think the "best" is pretty speculative, since different weapons had different roles. Best SMG? Best MG? ...and comparing one weapon to another is difficult because of specific ranges for the weapons. But as for hand-held weapons, I think the M2 Carbine was the best because it had a high rate of fire, was .30 cal, held up to 30 rounds depending on the mag, was very light (5.5 pounds) and had an effective range of up to 300 yards, and that's the distances that most of the fighting took place in WWII with hand-held weapons. It's kinda an all around weapon for close-to-medium combat. Unfortunately, it was not made in many numbers because the M1 Carbine had already been made in such great numbers.
PLease dont revive threads, WarWolves.