More than 50% of this forums discussion is Zoro vs Sanji.
Both fanbases constantly react to anything that has to do with the other character, such as the making of this thread.
The very design of these characters from them being the same age, and Zoro being pushed to be a single cm taller than Sanji both pre and both time skip.
Oda being asked in an SBS who's stronger and him publishing the question but diverting and not giving an answer to keep the discussion going.
Oda drawing paralels between Zoro and Sanji and a centuries old rivalry between the giants in Little Garden
They constantly tease each other when it comes to their strength and try to assert superiority, and it's not one sided, Zoro's just as sensitive about it :
Ask random One Piece readers about One Piece rivals and a significant majority will immediately think of Zoro and Sanji.
Zoro and Sanji compete for the same thing. That thing is bragging rights, and rubbing one's superiority in the other's face.
Whether is bounty, general strength, or whatever they can find in the spur of the moment to compete on, they are more than happy to try to one up the other, and salty whenever things don't go their way or the other teases them in regards to strength/bounty etc.
Like in the examples i've given above, Zoro is immediately shown to be sensitive when Sanji's strength is praised, or he's teased by Sanji with his currently superior bounty.
Their rivalry basically fits most this example from the definition i quoted
As they share a rivalry such as one would see between siblings.
It's not just a specific field that they compete in, but general strength/bounty/fame, and pretty much anything else they might come upon that allows them to compete with eachother, such as a hunting contest as the one i mentioned from back in Little Garden.