Luffy has many great fights:
Luffy vs Crocodile
Luffy vs Lucci
Luffy vs Doflamingo
Luffy vs Katakuri
Luffy vs Kaido
Out of these 5, I’d go with Kaido fight but other ones are also top notch.
Zoro’s case:
Zoro vs Mr. 1
Zoro vs King
I like Mr. 1 fight more but would not argue against anyone choosing King fight.
Sanji’s case:
Sanji vs Mr. 2
Sanji vs Jabra
Sanji vs Killingham
Killingham is BY FAR his best fight. New moves (Collier Fortresse, Sky Table Kick Course etc.), CoC/ACoC awakening, overall stat boosts, big character development, best choreography and the list goes on and on.
Gonna go with Luffy vs Kaido but all these listed fights are really amazing. Law+Kid vs BM is also rlly good as it re-explores the potential for DF based fights, especially Paramecias.