Luffy is factually the best fighter. He is at the top of the world being mostly self taught with no fighting style or weapon.
Sanji, Zoro and everyone esle where taught professionally, trained base physical strength, and have martial arts or OP weapons to fight.
Zoro was taught swordsmanship as a kid, trained physically, taught advanced haki concepts as a child, got OP Swords, trianed with the WSS in Sword techniques, and one Magic Sword( that allows him to do what Roger, Shanks, Big Mom anr Whitebeard failed to do).
Sanji was taught Swordsmandship, recieved physical training, martial arts, got Germa Genes, iffrit Jumble.
Luffy ONLY has his Devil Fruit( aka self taught) and Haki. Also fights barehanded.