Albert Barnes Commentary 1 Kings 16:30

"And Ahab the son of Omri did that which was evil in the sight of Jehovah above all that were before him." — 1 Kings 16:30 (ASV)

See 1 Kings 16:33. The great sin of Ahab—the one that set him apart from all his predecessors and made him exceed them in wickedness—was his introduction of Baal worship following his marriage to Jezebel, and his formal establishment of this blatant and tangible idolatry as the state religion.