In his fifth and final season with Iraklis, Diamantidis had his best year with the club. He averaged 14.8 points, 6.3 rebounds, 4.2 assists, a league-leading 2.5 steals, and 0.7 blocks per game, during the 2003–04 Greek League season. Due to his play, he was again voted to the Greek League's All-Star Game, and at the end of the season, he was also named the Greek League's MVP for the season. During his five seasons with Iraklis, one of the oldest Greek clubs, Diamantidis managed to help bring the team back into prominence in Greek competitions, after the club had been struggling in previous years.