Which king of Aram did Othniel son of Kenaz defeat?

Answer: Cushan-rishathaim

Israel served Cushan-rishathaim eight years. Then the Spirit of the LORD came on Othniel, who prevailed against him.

“Then the anger of the LORD burned against Israel, and He sold them into the hand of Cushan-rishathaim king of Aram-naharaim, and the Israelites served him eight years.”— Judges 3:8 (BSB)