What do you think was the best old world contribution to the new world and best new world contribution to the old world?

One of the best Old World contributions to the New World was the introduction of European agricultural practices and domesticated animals, which significantly transformed indigenous farming and livestock management. Conversely, the New World's best contribution to the Old World was the introduction of crops like potatoes, tomatoes, and maize, which became staples in European diets and had a profound impact on food security and population growth. Both exchanges fundamentally reshaped societies and economies on both sides of the Atlantic.