Since I watched One piece when I was a child, since I started reading the manga, I've always thought that Sanji was the third, in strength and hierarchy, Zoro is stronger and all, but I still feel like Sanji Is comparable to him, I'm not saying equal and not stronger, I just think that they're Luffy's strongest men who fought alongside the other for their captain, they're Luffy's wings and most trustworthy members of the crew.
I see people saying that Zoro is more comparable with Luffy than with Sanji, which I just think it's not true, even though Zoro Is clearly stronger and all I still see the cook being the same league, while Luffy being above.
Something like:
Luffy
---
Zoro
Sanji
---
The others