Why is Hashirama hailed as the most powerful shinobi of all time? Why is Hashirama hailed as the most powerful shinobi of all time and there hasn't been anyone who can surpass him? Also, why is the Wood Style unique to him?
There is more to it than this but the main reason he was considered the strongest was that his Wood Style allowed him to control the Tailed Beasts.  Before he did this there was no Jinchuuriki System.  The system only came after he passed them out to other Nations in good faith.  He potentially had all the Nine fighting for him at one point along with his own abilities.  

I think Wood Release was basically a Blood Release version of Senjutsu which could explain a lot of the shared attributes between the two (like extra damage, rapid healing, growing nature, etc) and was so overpowered. Imagine someone having access to massive nature chakra reserves at all time.  

I wish they explained Wood Style more and why the release was only unique to him and not his whole family (since it is considered a Kekkei Genkai).