My prefered WW2 weapon would be the atomic bomb. So far it has saved lives, as the only alternative to using said bomb would have been a full scale ground invasion of Japan... Costing millions of lives on both sides...
However, if "The Bomb" won't for some strange reason be counted as WW2 weapon, then I will go with the M1 Garand.
@Texcido, it was called a trench gun, and it was quite powerful and deadly in it's prime.
@Rubberychicken, the atomic bombs "Fatman" and "Little boy" are considered weapons. It's true however that it was neccesary to use such force as that the invasion of Japan would have cost millions of lives. Because Japan said that they would fight till their very dying breath.