Rashi (45b): He was more humble than the others, and exemplified "Ben Yechaved Av" (Mal'achi 1:6). When his father was a Metzora and Yosam judged the nation, he never took the crown, and every judgment he gave was in the name of his father.

R. Chananel (45b): Yosam, R. Shimon and his son did not receive any reward in this world for their merits.

'Rashi' (Divrei ha'Yamim II, 27:2 citing R. Elazar bar Moshe): Every other king has a sin. David - "Rak bi'Dvar Uriyah" (Shmuel II, 11:12). Shlomo - "Nashav Hitu Levavo" (Melachim I, 11:4). Rechav'am "Azav Es Toras Hashem" (Divrei ha'Yamim II, 12:1). Aviyah went "b'Chol Chat'os Aviv" (Melachim I, 15:3). Asa took the gold and silver from Otzeros Beis Hashem [to bribe Ben Hadad - ibid., 18] and incarcerated the Navi (above, 16:10). Yehoshafat joined with a Rasha (Melachim I, 22:48). Yehoram killed his brothers (Divrei ha'Yamim II, 21:4). Achazyahu followed his mother's counsel to do evil (above, 8:26). Yo'ash killed Zecharyah and became an idolatry (Divrei ha'Yamim II, 24:22). Amatzyah bowed to the idols of Se'ir (ibid., 25:14). Uziyah entered the Heichal to burn Ketores (ibid., 26:19). Achaz went in the way of Malchei Yisrael, and made Matzevos to Ba'alim (below, 16:3). Chizkiyah became haughty, and there was anger against him (Divrei ha'Yamim II, 32:25); Chachamim disagreed with three of the things he did (Pesachim 4:9). Menasheh was evil in Hashem's eyes (below, 21:2). Yoshiyah did not heed the Nevu'ah from Hashem (Divrei ha'Yamim II, 35:22). Tzidkiyah did evil in Hashem' eyes (below, 24:19), and incarcerated Yirmeyah (Yirmeyah 32:2-3). Yosam had no sin ¹ !