When I got into the thread, I had the impression that you will exclude Luffy and only focus on the underlings. but since Luffy is there, then the answer is obvious. It's him and no one else. There is a fucking reason why they follow him and not someone else.
However, if we exclude Luffy.
Then My answer will go to Jinbe (cause we already tested him leading half the SH against Big Mom) and we already saw him leading his Fishman Pirates, and how much he prioritizes his crew, also strong, intelligent, veteran, strategic...etc! And on top of that, if Zoro lead, Sanji will not like it, and if Sanji led, Zoro will also not like it.
And you shouldn't put Usopp as captain despite his good leadership in some moments or Nami, because the boss needs to honestly be strong and hold his own in big fights, so Jinbe is the most suitable after Luffy in my opinion.
Of course that's if you want the crewmembers to stay the same. But if you are someone who doesn't care about crewmembers leaving, then you can make Zoro or Sanji the new leader and let the other person leave just because your ego doesn't allow you to put your bias on the side and choose someone who others will follow without complaint
However, if we exclude Luffy.
Then My answer will go to Jinbe (cause we already tested him leading half the SH against Big Mom) and we already saw him leading his Fishman Pirates, and how much he prioritizes his crew, also strong, intelligent, veteran, strategic...etc! And on top of that, if Zoro lead, Sanji will not like it, and if Sanji led, Zoro will also not like it.
And you shouldn't put Usopp as captain despite his good leadership in some moments or Nami, because the boss needs to honestly be strong and hold his own in big fights, so Jinbe is the most suitable after Luffy in my opinion.
Of course that's if you want the crewmembers to stay the same. But if you are someone who doesn't care about crewmembers leaving, then you can make Zoro or Sanji the new leader and let the other person leave just because your ego doesn't allow you to put your bias on the side and choose someone who others will follow without complaint