No matter where Kageyama goes, what international clubs or superstars of the sport he plays with, to him, the best volleyball player in the world will always be Oikawa Tooru.

Oikawa sighs forlornly into his coffee, lamenting the fact that he's not the one sharing meat buns with Tobio-chan.
(Tsukishima aggressively pretends he doesn't know either Oikawa or Kageyama)

Kageyama totally wears black volleyball shoes because he saw Oikawa wearing them at Kitagawa Daiichi.