He can be the strongest warrior in China if he's going to be Shin's EoS fight. For which there is a good amount of set up for. We know hara is gonna be going with the SHK betrayal story, where he betrays Qin and turns to the Chu. After turning to the Chu he raises rebellions in all the Chu cities Shin conquered (as Shin progresses further and further taking Ws in Chu). Eventually Kouen ambushes the HSU, during that battle, Shouheikun comes in and pincers him. Which results in Shin losing some of his notable officers.
So in this narrative Shin's biggest L will happen due to Shouheikun's betrayal. He will lose his top officers (who are also essentially his closest friends)...due to Shouheikun's betrayal.
Now let's review SHK & Shin's story so far:
-> Shouheikun wanted Shin the most out of all the youngins.
-> Shouheikun even wanted to give Shin the opportunity for Riboku
-> Shouheikun has consistantly kept a close eye on Shin's progression (literally and figuratively)
-> Shouheikun even provided HSU with the funds to boost their prowess.
-> He allowed Kyoukai to continue to stay with the HSU as Shin's Lt.
Point being that there is a connection being built between the two over the course of the manga... and that connection ends in the betrayal by SHK, in which he destroys the very unit he helped build by providing favorable support to it. The betrayal would then help set up Shin vs Shouheikun 1on1 in the 2nd Chu invasion under Ousen. And if that is the case.. then Shouheikun at his peak/prime would have to be the strongest in China, BECAUSE Shin's goal isn't just to be the greatest of the generals under the heavens, it's also to be the strongest.
And to add on something to Shouheikun, we might actually see him pull something off like Ei Sei, when he causes rebellions in the Chu cities Shin conquers.
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So in the end the strongest combatant in China could very well end up between Shouheikun & Kouen. Although it's also possible that it might the kid from Chu. But yea.
He has the potential to be the strongest warrior in China in theory. But I just don't know whether Hara would make it happen.. and especially if that's the case right now at this very moment.