In my opinion Zoro and Sanji are underrated only because of their monstrous and charismatic captain Monkey D. Luffy.
There’s no debate that Luffy is physically stronger and more capable than both Zoro and Sanji in almost every category imaginable – strength, durability, speed, willpower, Haki.
Basically almost any physical feat they can muster, Luffy will more than likely outdo them. However, Luffy is a beast among monsters so even being compared to him is a feat in and of itself. Comparatively speaking Luffy is ahead, but they’re always just a tier below him in my opinion.
Unfortunately, neither Zoro or Sanji has had any serious fights since the timeskip. Zoro pretty much trivializes nearly every opponent he’s come across. He has recently defeated a fellow Supernova (although we have to acknowledge Killer was under distress) and performed impressively against Hawkins.
Sanji on the other hand has only had brief clashes, but with very strong opponents – Vergo, Doflamingo, Oven, Daifuku, Page One, King and to be honest, Doflamingo was the only one who kicked his ass.
Then on Whole Cake Island, Sanji repeatedly clashed with Oven and Daifuku whom people seem to forget are Katakuri’s twin brothers. Now, I’m not saying they’re even remotely on Katakuri’s level, but they do seem to be the strongest of Big Mom’s crew outside of the Sweet Commanders. I think they’re stronger than Perospero, but that’s debatable I guess.
Therefore, if Luffy is currently first Yonko Commander level or above, Zoro and Sanji should at the very least be YC2-YC3 level, which would make them as strong as Ace/Jinbe were. Just think about that. Not to mention, at the end of the series if there is presumably a war against the World Government I strongly believe Zoro and Sanji will have to fight or even defeat Admirals. Possibly Zoro vs Fujitora and Sanji vs Kizaru.
*Theory by RB8Gem9
