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).
The reason that Hashirama is the only shinobi to don the wood release is purely due to genetics and that alone. Don't quote me exactly on this, however there is a 0.0000001% change of genetically developing mokuton and Yamato was lucky enough to get it through experimentation.
As for why he's considered the strongest, I'd assume it's due to the nature of the wood release, it's classed as a kekkei genkei and is extremely rare, it can suppress the tailed beasts (as shown in episodes such as Naruto's training with Yamato and Kakashi), however it's not just that alone, I'd assume it's also due to his good nature, his determination and the sheer fact he genuinely worked extremely hard to get to the position he was in.