Need to define seriousness within rivalry, though.
If banter is what constitutes serious rivalry, then Luffy and Buggy had a lot of it, as well.
Do they set each other up as strength goals to achieve like Zoro and Kuina? Not really.
Is surpassing or defeating one another a serious, genuine goal of theirs? Not really no, they barely give a fuck about that when thinking about getting stronger lol.
Do their dreams and end goals clash in a competition where one must beat another? Like Luffy and Kidd both aiming for same end game? Not really, no.
Bickering and some silly contests don't constitute a serious rivalry, without any lack of real character investments or character goal alignment.
Hell, if "Zoro and Sanji rivalry" is supposed to be serious, then what was Kuina and Zoro? Ultra omega saiyan serious?
And ofc Zoro thinks highly of Sanji
@Yasheen He thinks highly of Luffy, too- and his nakama in general, even if it's not strength-based view with likes of Sanji. But the word "serious" is being used too lightly here.