Albert Barnes Commentary 2 Kings 18:5

"He trusted in Jehovah, the God of Israel; so that after him was none like him among all the kings of Judah, nor [among them] that were before him." — 2 Kings 18:5 (ASV)

After him was none like him - The same is said of Josiah (marginal reference). The phrase was probably proverbial and was not taken to mean more than what we mean when we say that such a king was one of singular piety.