Which Assyrian king threatened Jerusalem during Hezekiah's reign?

Answer: Sennacherib

Sennacherib king of Assyria attacked all the fortified cities of Judah.

“In the fourteenth year of Hezekiah’s reign, Sennacherib king of Assyria attacked and captured all the fortified cities of Judah.”— 2 Kings 18:13 (BSB)