The Holocaust overshadows most genocides because it was on a such large scale. But yes, the Great War had it's share of atrocities, such as "the Rape of Belgium" where German forces would treat Belgian civilians as guerilla fighters and murder them in the streets. Wether this was genocide is heavily debated, as there is evidence that the Germans acted out of paranoia. A Companion to World War I by John Home. Events during the Armenian Genocide occured during World War I, but the crimes against humanity were not caused by World War I, they were caused by the Balkan Wars and had origins beyond the War.

World War I is also the first instance of widespread use of chemical weapons. While only 4% of casualties in World War I came from poison gas, under the 1899 Hague Declaration Concerning Asphyxiating Gases and the 1907 Hague Convention on Land Warfare it was considered a war crime. Same source used.

I don't believe in "keeping scores" in genocide and war, but by statistics, the World War II crimes are for certain larger.. In the span of 7-8 years you have the Holocaust and Japanese atrocities, including "comfort women", essentially women pressed into prostitution, as well as the various death marches and death camps used by the Japanese.