That's the thing. Potentially. Potentially, there are a lot of players that a lot of clubs sign and have (And City themselves have many) who could be great.
But based on ACTUAL performances which is what determines how good, useful or effective a player is, Robinho hasn't shown much apart from when sporting the Brazil jersey and even then not always at his best. I wasn't impressed with him at Real Madrid either though his final season was better than the previous ones. He's got the same ego as some of the world's greatest players (Robben, Ronaldo, Ribery, etc. ) but not the same ability.
Based on actual performances which is what determines a player's worth and rank and usefulness for me, he would be a rotation player at Man City. Last I recall, he couldn't beat Craig Bellamy for a starting line up position and Craig is not exactly a world-beater. And soon he'll have to compete with a lot more than just Craig Bellamy. He already does as City have gotten stronger since he left and will continue to do so as they bring better and better players with their $$$.
Tevez is clearly a far better player and for me he's not even the only one. Is Robinho better than Given? De Jong? etc. And now Yaya and Silva? Well for me, no because they give a lot more to team and are more effective but of course it's always hard to compare players of different positions/skill-set anyway.
Robinho is talented, no question about that. But he's moody, selfish, inconsistent and has shitty attitude which means more often than not, you see him play only at a fraction level of what he's capable of (Which is why I understand what you mean by potentially and I agree with that but that doesn't mean much does it? ) especially since he's not so young anymore, so it's not like we're talking about some 18-year old talent who will grow up and become mature and professional or get faster/stronger.
Adam Johnson is already a pretty good player and potentially he could be their best too. If anything, someone like him is young and probably WILL get better. Robinho has to worry about not declining in ability just as much if not more than further improving at this age, rate and level of professionalism.