Albert Barnes Commentary 2 Kings 18:19

"And Rabshakeh said unto them, Say ye now to Hezekiah, Thus saith the great king, the king of Assyria, What confidence is this wherein thou trustest?" — 2 Kings 18:19 (ASV)

The Rab-shakeh, third in rank among the three Assyrian ambassadors, likely took the prominent part in the conference because he could speak Hebrew (2 Kings 18:26), whereas the Tartan and the Rabsaris could not.

The great king – This title was used by the monarchs of Assyria as early as 1120 B.C. Like the title "king of kings," the distinctive epithet "great" served to emphatically mark the vast difference between the numerous vassal monarchs and the suzerain from whom they held their crowns.