I am pretty sure it was stated that Zoro is the 2nd strongest member of Luffy's crew. I Don't know exactly when or where it was stated but that doesn't really matter honestly. It's obvious if you have read one piece from the beginning that Zoro always fought the 2nd strongest person and Sanji the 3rd strongest. They definitely have a rivalry going on but it's clear that Zoro is no doubt about it above Sanji in terms of strength. Not cooking of course
I don't know why people are arguing this haha. I'm a Sanji fan and I do recognize Zoro as stronger, but difference is I think he's only SLIGHTLY above. and I honestly feel the only difference that's making Zoro have a slight edge over Sanji is simply his determination and will which does NOT WAVER at all when he's fighting. His focus is always to win and not accept defeat even if it means death
While, Sanji's mentality is somewhat different in that he would accept a defeat and even stake his pride for the sake of living another day to fight for another day. This kind of different mentality makes Zoro being pushed more, resulting in showing MORE feats and strength wise.
Hell, you can even go back to Baratie arc, and witness the dialogs between Sanji and Zeff about Luffy's unwavering will and his determination of betting his life on the line in the fight. Or even go and see Zoro vs Mihawk and see what Sanji suggested to "throw away your ambition" after Zoro was cut by Mihawk, because in Sanji's mentality, he's somewhat more about living for another day and fight for another day.
Honestly, I feel that's what makes Zoro more tenacious which really makes Zoro stands out slightly more than Sanji
HOWEVER, who is the one character Sanji refuses to actually give up against??? It's Zoro when they face each others, that's why EVEN THOUGH Zoro might have shown us better feats, Sanji when he clashes with Zoro even in comedy scenes, or when they compete against enemies, they appear somewhat equal cause Sanji's will and determination in those moments are equal to Zoro of refusing to lose at all cost against each other.
But when facing other enemies, I feel Zoro sustains this attitude, while Sanji does NOT and I think Zoro sustains it simply because of his promise to Luffy of not losing again, while also his promise to be WSS with kuina. On the other hand, Sanji's dream will only be accomplished if he traveled more until finds all blue, so his survival comes first and before his strength for his ambition, while with Zoro his ambition is strength related, and his promise to Luffy makes him not give up as easily as Sanji.