Psalm 88 is titled as a Maskil of which man?

Answer: Heman the Ezrahite

Psalm 88's title calls it a psalm of the sons of Korah and "A Maskil of Heman the Ezrahite." Psalm 89 is a Maskil of Ethan the Ezrahite.

“A song. A Psalm of the sons of Korah. For the choirmaster. According to Mahalath Leannoth. A Maskil of Heman the Ezrahite. O LORD, the God of my salvation, day and night I cry out before You.”— Psalm 88:1 (BSB)