One of the most eternal debates among One Piece fans has remained: who is stronger, Roronoa Zoro or Sanji? Such a rivalry between the swordsman and cook of the Straw Hat Pirates has been running throughout the series, intertwining to furnish tension and brotherhood within the crew.
Comparing the strengths of Zoro and Sanji, it becomes evident that the two characters specialize in different things. Zoro has consistently demonstrated more raw strength, often doing great physical deeds. His fighting style has been epitomized by carrying three swords and cutting through supposedly indestructible materials. Sanji has better speed and agility and has a leg-based combat system which generally provides fast but powerful strikes.
Both characters have been very enduring and full of stamina throughout the series, so it is pretty hard to give a clear advantage in that area. The ability to take massive amounts of damage and still keep on fighting has been what decided the victory of many of the harder battles the Straw Hat Pirates have fought.
Although many fans and analysts do believe that Zoro is a little better in overall fighting capacity, the margin between the two is often assessed to be very small. This close comparison in strength has been a deliberate aspect of their characterization by Eiichiro Oda, series creator, maintaining a sense of rivalry or competition between them.
Some arguments on Zoro's side of being the strongest include when certain feats are pointed out, such as the ability to throw incredibly heavy objects; for instance, he could throw a horn that was "infinitely heavier than Queen" off an island. Often, displays of raw power like these have been used to argue Zoro's side in favor of being the superior combatant.
The strengths of Sanji, however, lie elsewhere. He is fast, a tactical thinker, and possesses other unique abilities, including turning invisible and the "Diable Jambe" techniques, which make him a formidable fighter in his own right. More dimensions were brought into the way Sanji fights with the introduction of Raid Suit.
Recent developments within the manga have still spiced up this debate on Zoro versus Sanji's strength. Recent chapters tease that both characters are stronger than ever, improving their growth and development as parallel to each other. Their continuous growth of abilities keeps this comparison alive and relevant.
This is where one should realize that their strength comparison is not in raw power or fighting ability between Zoro and Sanji. They are both working under really different roles and yet very crucial to the Straw Hats. Thus, calling them the "Wings of the Pirate King" reflects that their strengths complement one another rather than existing in direct competition.
The rivalry between Zoro and Sanji does not truly stop at physical strength. Rather, it is to be found in very different personalities and methods of solving problems, as well as the way to work with other members of their crew. In a sense, this complex relationship creates depth in their individual personalities, serving both as a source for conflict and humor within the series.
While many fans like to argue over who would win if Zoro and Sanji were to engage in a straight-up fight, the truth is that is unlikely to ever happen in the course of the story. Their mutual loyalty to Luffy and the crew forbids any kind of serious fight between the two, along with the buried respect they hold for each other despite the constant bickering.