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I know that right now it looks like Zoro is much stronger than Sanji, but bear with me. This theory stems from something that happened lot of time ago, precisely during the Little Garden Arc.
In the beginning of the arc, Zoro and Sanji both went hunting and ended up arguing about who got the biggest prey while they were pretty much the same size. Fast forward at the end of the arc, it is revelead Dorry and Brogy started arguing for the same reason – who killed the biggest Sea King – and from then they fought countless duels without ever finding a winner.
This is Oda’s concept of rivals. Just like Whitebeard and Roger or Roger and Garp fought countless times without nobody ever winning.
Till the timeskip, this holds perfectly. They are always arguing about their bounties and they almost always battle similar strenght opponents: Daz Bones and Bon Chan, Kaku and Jabura and so on.
After timeskip, things are fuzzy but – unpopular opinion, i know – we can’t 100% say Zoro is superior. The greenhead always won, but he never fought the strongest fighters of the arc, fighting against Monet and Pica.
Sanji on the other hand always ended up in difficult situations, fighting with handicaps (Punk Hazard), against the strongest fighters of the arc (Vergo, Doflamingo, various spars on Whole Cake) or losing to dirty tricks (Judge). Even if Zoro is currently superior, Sanji got a major power up lately with the Raid Suit.
Conclusion: Zoro and Sanji will end up being exactly at the same level, i think they will both defeat an Admiral on their own (Fujitora and Kizaru, i guess).
I’d love if at the end of One Piece, they will end up with the same bounty and decide to have a duel to settle things, but just like Dorry and Brogy no one will win. After each duel, Zoro will go hunting and Sanji will cook the food, then they eat together constantly arguing like always. It would be an amazing bromance.
*Theory by Handsome_Claptrap