It's most certainly NOT the atomic bombs.
Between the two bombs dropped on Japan, there were likely considerably less than 300,000 casualties. This is a terrible number of people killed and in pain, but it pales in comparison to the 50,000,000 killed worldwide during the war, with as many as half of those being Russians.
There may not be any hard and fast answer to this, but some good guesses would be certain German small arms such as the MG-34, or MG-42 machine guns, the MP-40 submachine gun, or the standard infantry rifle Mauser k98, which saw action in both world wars.
The british Lee-Enfield saw action in both wars as well.
The nuclear bomb
Atomicbomb.
Probably artillery; it was responsible for many of the deaths in World War 1.
8 million horses were killed in world war 1
Big guns (i.e. artillery, not hand weapons).
The 30.cal machine gun.
Yes, it killed many people.
There can be no doubt that the most deadliest weapon of world war 2 was the nuclear bombs that dropped on Hiroshima.
the tank
Propaganda.
Mustard Gas
The most common one that was used as a weapon in World War 1 is called the Artillery
The most common method was with artillery.It is NOT a biological weapon, it is a chemical weapon.
The Atomic bomb.
Atomicbomb.
The A-bomb.
The nuclear bomb