Who was the longest-reigning Byzantine Emperor?

The longest-reigning Byzantine Emperor was Emperor Constantine X Doukas, who ruled from 1059 to 1067 CE. He came to the throne after the previous emperor, Isaac I Komnenos, abdicated due to illness. Constantine X was the founder of the Doukid dynasty, which ruled the Byzantine Empire from 1059 to 1081 CE.

During his reign, Constantine X was faced with a number of challenges, including a military revolt in the Balkans and a series of attacks by the Seljuk Turks in Asia Minor. He attempted to reform the administration of the Empire and strengthen the military, but was ultimately unsuccessful in his efforts.

Constantine X was succeeded by his son, Michael VII Doukas, who was deposed by the future emperor Nikephoros III Botaneiates in 1078 CE. The Doukid dynasty came to an end in 1081 CE, when the general Alexios Komnenos seized the throne.