World War I
World War II
The ending of World War One (The Treaty of Versailles) was the leading factor of World War Two
As the previous commented has remarked, the Second World War would have not been if the First World War had never happened. Because they are so interrelated, the Great War causes the Second World War, thus causing more damage than it would have. Sure, you can make this arguement about any historical event, but because WW2 was a direct result of WW1 (a cause) then the latter did more damage overall.
While more terrible things happened in WW2, WW1 paved the way for all of those things and basically caused it.
It probably had more than double the casualties of the first one and was the most devastating war in history. Shame us Italians allied with the wrong side!
While the futility of WWI is terrible, the mass bombing campaigns of WWII did much more damage.