If he can hide his aura and turn invisible, how the hell is anyone suppose to beat him?...If Zoro does manage to pull it off, he automatically becomes the best CoO user but until then my money is on Sanji ending up as the best CoO user.
He might not be the first mate officially, but he fits and act like one...He was the first to join, undoubtly the most loyal to Luffy, bodychecks the rest and enforces the fact that Luffy is the captain
Nobody can beat him except the only character who ever has shown the ability to detect him - hearing the breath of things.
He could mask his aura but Zoro will still hear the breath/heartbeat of either him or the steel of his sword.
It's not automatically making Zoro into best CoO user but certainly makes him a candidate.
One thing is certain, nobody else would be able to beat Shiryu in such case because other branches of CoO dont help in this case.
Being the best in one branch of CoO doesnt make you the best CoO user overall, it's hard to tell who is the best.
Sanji may have another separated branch of CoO in which he will be good but it's not FS or breath of things.
Yes, he fits and acts as one but Oda never wants to pin Zoro down and make him into a clear second because it is unfitting.
Zoro has his own values for what a captain represents and as long as Luffy is fulfilling those, he doesn't intervene in those matters.
Only when Luffy steps out of the line, Zoro corrects him. Ultimately, Zoro is in the crew because he gave his word because Luffy saved him. He has long repaid the debt but he still intends to honor the promise. We will see what happens after Luffy has become PK.