To me, hands down WWII. The massive executions of civilians to go with the massive deaths and horrid conditions in Russia are what made the difference to me. WWI was civilized military on military with civilians being killed but not purposely targeted like they were in WWII. WWI saw ~1.75% of the population killed while WWII saw ~3.5% killed. The total deaths on civilians in WWII by military (Crimes against humanity included) were around 30,000,000 while WWI had around 2,000,000 deaths in the same category.
The conditions of those two wars were the worst in human history in my opinion.