Which contribution by a daimyo was the most significant? why?

1. The biggest contribution of the daimyo's was the that Japan was finally united. Before Japan was unified, there was a Sengoku period, that is a period of constant fighting that lasted for many years. It was not until Tokugawa Ieyasu managed to end this period and establish Tokugawa Shogunate. 

2. Tokugawa Ieyasu's feat was really important because he ended a long period of fighting. He also began the Tokugawa reign that lasted about two centuries. He also moved the capital to Edo from Kyoto and politically organized the country. This type of organization lasted until 19th century.