rjh01
Gentleman of leisure
I'm not sure such horrors deserve actual ranking, but if you want to go by numbers, the mass murder of the Jews by the Nazis is still up at the top in that respect.
Mass murder by numbers here: http://en.wikipedia.org/wiki/List_of_genocides_by_death_toll
If that list was in order then second on the list would be Holodomor. Between 2.4 and 7.5 million people starved to death in about a year. Hitler took several years to kill maybe double the number of people.