It's hard to say Toure is the best player in the league. He is above average in many different areas and has excelled as an attacking mid, holding mid, basically anywhere in the midfield. But both Toure and Silva are helped by having Tevez and Aguero playing up top because both those guys make any midfielders look better.
If we took last season alone it is hard not to say that Adebayor or RVP were much more influential on matches than just Silva or Toure. Then if we add Mata into the mix, Mata was better than Silva last year in the league on a team that didn't get near the results. Put a top top striker last year on Chelsea and Mata get much more recognition.
If we look at deeper playmakers which there is no doubt Yaya can play the role, Modric, Arteta, Song, maybe even Cabaye can be more effective.
I think Yaya is excellent, and clearly a top player but to me his talent is showcased even less at City because Aguero, Tevez, Dzeko are all guys who can bag 20+ goals regardless of their midfield behind them.