The emperor with the longest lifespan and greatest contribution in Chinese history is Kangxi Emperor, the fifth emperor of China's Qing Dynasty. Kangxi Emperor ruled for 61 years and lived from 1654 to 1722.
Some of his greatest contributions include his efforts to modernize the country, support for various fields such as literature, science, mathematics, and philosophy, development of international trade, and promotion of foreign language learning. Additionally, he expanded China's territories by including Tibet and East Turkestan in the west.
Kangxi Emperor is one of the longest-reigning emperors in Chinese history and is highly respected for his contributions to China's modernization.