This is, of course, a question that cannot be answered objectively. There are many possible answers. I would argue that there are two answers that are better than others.
First, the conduct of the war brought about the beginning of the concept of total war. This was the first war in which essentially the entire society had to be mobilized to participate in the war effort. It was the first war in which the populations of the combatants were attacked because of their role in the war effort. This, along with the rise in industrial warfare (using tanks, aircraft, submarines, and machine guns, among other things), was a major legacy of this war.
Second, the resolution of the war brought about WWII. It also helped to bring about the Holocaust, which is arguably the worst thing that Western civilization ever did. The Treaty of Versailles helped cause tremendous anger in Germany. This anger helped lead to the rise of Hitler and the Nazis which, of course, brought the Holocaust about.
These two impacts of WWI were the most significant effects of that war on Western civilization.