Not Hitler's genocide, nor Stalin's, nor Mao's. Based on a comparison of the number of unjust deaths, the clear winner is the global ban of the insecticide DDT. Pretty much every scientific claim made to support the ban has been thoroughly refuted, and now Rachel Carson along with her fellow travelers can claim credit for tens of millions of deaths from malaria, a disease that had pretty much been eradicated in the Western world at the time the ban was imposed.
Despite the overwhelming scientific evidence, nobody of consequence dares to call for the lifting of the ban. And the death toll continues to mount.