Pre Zou arc, I would have said Zoro but Zou proved just how terrible of a leader Zoro would be.
If he started a crew, it will end as badly as the Rocks Pirates falling appart from the inside out.
Zoro is good at fighting but is a completely and utter failure when it comes to anything social.
Zoro is a meathead( this is good to have on a crew), atleast 1. This is a warrior that is the trigger man for the Captain.
Sanji is a better leader than Zoro as he is more in tune with the emotions of other people, and is more tactical in his approach.
Baratie Arc shows some of Sanji's leadership skills.