But it's not clear that Zoro is the undisputable #2?
Either way, not 1 time in the whole series have Zoro and Sanji had a legitimate brawl or serious strength contest. It is almost always just back-and-forth bickering and trying to make the other guy look bad. Luffy & Zoro have a significantly friendlier relationship than Zoro & Sanji, so there's no real reason for them to randomly break out into arguments.
Plus, Zoro & Luffy are both part of the Worst Generation, who are all collectively called "rivals" for being the top dog of the pirate world. That's a cheap argument though, so I'm not gonna go further down that road