Was he greater than all after him? It says about Yoshiyah "v'Chamohu Lo Hayah Lefanav"!
Malbim: Chizkiyah was greater in Bitachon. Yoshiyah was greater in Teshuvah 1 .
Malbim (23:25, citing Mahari): Even according to the opinion that Yoshiyah did not sin, he overturned every judgment he made before 18, and paid from his money. Malbim - he made the generation repent from all the abominations. He abolished the Mizbechos and Bamos from all cities of Yisrael.
Was he greater than all before him, including David?
Radak: He was greater than those before him, except for David and Shlomo. Even though there good kings before him - Asa, Yehoshafat, Yo'ash in the days of Yehoyada, Uziyah and Yosam - they were not as straight in Hashem's eyes as he was. He did "k'Chol Asher Asah David Aviv" (verse 3)!
