Powermetal4ever, there will never be anything like "the last days" in the Bible. We can have major catastrophes with 90% of our population killed but that would not be Armageddon or any other aspect of the Biblical predictions and interpretations thereof.
As for playing the game of which disaster is the worse my two favorites are the Black Death (Bubonic plague) and the Taiping rebellion.
The Bubonic/Pneumonic plague pandemics of the 14th century killed 1/4 of the population throughout Eurasia in the first sweep. There were four other pandemics in a 100 year period. I mention this example not merely because of its large numbers but because of the large percentage of the population affected.
The Taiping Rebellion was a civil war in China in 1850-1864 that killed 20 million people. I like to use this as an example because even though more people died in World War II this war happened before 1914. That should throw the Witnesses off balance.