In my opinion, Haikyuu!! deserves the <Best Character Development Award> out of all the anime that I have watched this year.
Every single episode, at least one character grows mentally. Kageyama has been shown to have grown the most, but a lot of other characters have changed for the better over the course of the anime. To list a few, Asahi got over his fear of spiking, Tsukishima is training to serve, and Hinata has learned to be more versatile when spiking. ((I have posted about these examples previously, select the Category #Haikyuu!! to see them :3 ))
And, I would also like to point out that Haikyuu!! did not only shine the light on Karasuno, but also managed to highlight the more important characters in other teams, like Oikawa in Seijoh, and Kenma in Nekoma, though not so much for the latter.
Lastly, if you notice, the anime does not directly say out the growth of the characters. Imagery is always used to indirectly portray the changes. This is a really creative method. Some examples would be when Kageyama evolved from being a King to a fellow teammate, the time when Karasuno was fighting against Dateko High and they broke through the wall, etc…
Of course, this is just my opinion, so feel free to share what you think :3