Zoro is the first mate as stated by multiple characters in the series such as Urogue and Bartolomeo.
Nami is Luffy's left hand, not the guy who beat him up in enemy territory.
Urouge never, ever mentions the concept of "first mate" in Japanese.
Bartolomeo's statement gets added a "(by me)", so it's even stressed by Oda to be a subjective assessment by a fanboy (wouldn't even surprise me the author were mocking you all).
Zoro's official role is combatant as per Vivre Card, like Lucky Roo but unlike Benn Beckman. "First mate" is an official title that only a few "right hand men" share. I stated a fact, yours is just an opinion.
"The guy who beat him up in enemy territory" yet both Luffy and Sanji were perfectly aware of the circumstances leading to that situation. Some of you should accept for once that Luffy doesn't rule his crew as you'd wish in your wet and autocratic dreams and that he's an empathetic leader that takes an approach based on interdependence. Precisely, in that moment it was Nami, your so-called "left hand", who made the mistake of not understanding the circumstances as quick and clearly as Luffy did from the start.