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.
I believe Natsume Takashi is not the strongest spiritually as there are people like Matoba and his grandma. But when it comes down to battling with other exorcists or handling yokai business, he is the strongest. Because there are many yokai and gods which he made friends with, that will come to his aid. Monsters like Madara, Hinoe and that huge cow like yoki (forget his name) are high class.Not to mention he as a book of friend, which he can summon yokai to assist him. So when its comes to pure power or throwing hands with someone, he is unstoppable. He got too many powerfully allies to call help upon if he wants and nobody would survive if all strong yokai gang up, not even Motaba will stand a chance.
I think spiritually Natsume Takashi is strong. As already mentioned, due to his nature he usually avoids conflicts. He also isn't really physically fit and doesn't know much about yokai, exorcists and exorcism techniques. Though if he did seriously decide to go against either exorcists or yokai I think he would be a strong rival.
Explanation with reference ahead:
It's already known that Matoba Seiji is stronger than Natori Shuuichi as shown in the flashback. At their first exorcist meeting, Shuuichi saw a dark red coloured kimono hung from a tree. The colour one saw symbolized the extent of their powers, truly powerful ones even saw patterns, which meant Shuuichi was above average. But it is said that Seiji was an exceptional exorcist.
Also! In the "white mist village" arc, in Vol.25 I think, Natsume could clearly see the talisman floating in the sky but Matoba Seiji could only see it when he strained his eyes (to what extent we'd never know) and Natori Shuuichi wasn't able to see it at all. This perfectly compares their "spiritual" power I think.