It's always going to be Luffy>Zoro>Sanji. I mean Shonen law dictates that the protagonist is the strongest. Luffy also has legendary genes, 3 types of haki (has he unlocked all 3 advanced forms or just 2?), a Devil Fruit that was recently revealed to be a fruit wanted by the WG, and he's been training since childhood. Luffy needs Zoro to become stronger.
Zoro has haki and he has been training since childhood. We can also argue that he is an expert swordsman. The difference is their personality. Luffy is adventurous and carefree. Oda uses Luffy to help establish the exposition to the readers. It's usually through Luffy that we see the civilians and their struggles.
Zoro on the other hand, is more analytical. He's rational and is able to identify potential threats. Also I think Luffy likes exploration more than fighting while Zoro puts fighting first and foremost. Zoro is the one who helps Luffy assess different situations. Luffy knows he can enjoy himself and not worry about the crew while Zoro is there. But as soon as Zoro says x person or situation is serious then Luffy listens. If Luffy sees Zoro struggling or injured in a fight, then he knows that he can't fool around anymore. That is when he starts to get serious.
In conclusion I see Luffy as someone who has a world of potential but doesn't always use it. Meanwhile, Zoro is strong because he pushes himself beyond his limits. While Luffy is stronger and always will be, he relies on Zoro to assess situations. That is one of the many things that makes their dynamic amazing.