Which Fire Lord began the Hundred Year War?

Fire Lord Sozin began the Hundred Year War.

Sozin was the Fire Lord who launched the Fire Nation's campaign of imperial expansion. He used the power of the great comet later named after him to intensify the Fire Nation's attack and began the genocide of the Air Nomads, setting the central conflict of Avatar: The Last Airbender in motion.

Before becoming an enemy of the world, Sozin had been Avatar Roku's childhood friend. Roku, the Avatar of Sozin's generation, opposed his ambition to spread Fire Nation influence. Their relationship deteriorated when Sozin refused to abandon his plans for conquest. After Roku died, Sozin used the Avatar's absence to expand the war.

The Hundred Year War continued through the reigns of later Fire Lords, including Azulon and Ozai. Ozai was the ruler defeated by Aang at the end of the original series, but he did not start the war. Zoryu is a later Fire Nation historical figure, not the conflict's initiator.