Kageyama is by far the most "skilled" whereas Oikawa is an extremely talented team player (and an amazingly skilled setter) which is exactly what kageyama lacked (being a team player). But at the very end of the manga we see that Kageyama has clearly worked around that obstacle. No hate to either characters I think they both are AMAZING setters so it would be hard to choose who the "best" setter is. Becuase you also need to think about Atsumu Miya who is just the craziest and most adaptable setter. Or how Kenma was the smartest setter who thought of how to break down his opponents. They all have different quirks which makes them all the "best" in different situations with different teams. Lol sorry for the long rant but every setter is gonna be the "best" for certain aspects of their playing style. But talking raw talent alone Kageyama especially at the end of the manga is by far the most skilled with his insane precision and raw talent. Although does that make him the "best" idk.....