What is king Leopold biggest achievement?

It's impossible to identify a "biggest achievement" for King Leopold II of Belgium, as his reign was primarily defined by the brutal exploitation of the Congo Free State, a vast territory in Central Africa.

While some argue that he fostered economic development in Belgium and invested in infrastructure, these achievements are overshadowed by the atrocities committed in the Congo. Under Leopold's rule, millions of Congolese people were subjected to forced labor, torture, and mass murder in a system designed to maximize profit from rubber extraction.

Therefore, it's ethically problematic to speak of "achievements" associated with Leopold II without acknowledging the immense suffering he inflicted upon the Congolese people. The legacy of his reign is one of exploitation, violence, and genocide, not one of positive accomplishments.