Which king of Judah allied with Ahab against Aram?

Answer: Jehoshaphat

Jehoshaphat king of Judah allied with Ahab to fight against Ramoth Gilead.

“So he asked Jehoshaphat, “Will you go with me to fight against Ramoth-gilead?” Jehoshaphat answered the king of Israel, “I am as you are, my people are your people, and my horses are your horses.””— 1 Kings 22:4 (BSB)