How strong is Natsume Takashi? It is said that Natsume Takashi is strong by many youkai in instances, but Matoba and Natori also seem strong, so is he the strongest?
I think Takashi is strong.  Maybe not the strongest, but certainly very strong.  

There are several hints, when he does resort to violence or manages to break seals.  He may be as powerful as Reiko was.  However, due to his nature he tends to avoid conflict and never sees himself as being formidable.  As such, we never see the full extent of his potential.