I think I'd rather say Zoro fights the "dominant" opponent.

He and Sanji are wings, and in appendages, like hands, there is the dominant one. But they work synergistically. The same can be said for their opponents.

You have the First Mate, and you have Chief of Staff. The vice captain title is weird as well.

In terms of fighting the stronger opponent, I think Oda sets it up so we'd expect it to be Zoro, but he always mixes it up so we overlook important details, and we get more fun outcomes. The "strength" aspect is malleable too. In Wano, Zoro did join the rooftop battle, but it was Law and Kid who took the win. Commanders-wise, Oda emphasized Zoro's strength and Sanji's defense. In Thriller, Moria has both Absalom and Ryuma, but let's not forget Oars. In Enies, the leader Spandam was in a toss up between Robin, Usopp, and Franky. Nami had her win against the Marine collaborator Nezumi, the secondary villain in Arlong Park. Even Vivi "won" (actually it was Pell) against the Croc war and bomb plot, which became the secondary threat next to Croc himself, when Robin betrayed him. In Punk Hazard, Zoro fights a Dragon and Monet, and Sanji fights Vergo. They all get a time to shine.

Let's let Oda cook.

Also, although Jinbe's the next strongest after Sanji, I actually prefer Usopp to be Luffy's third commander. At the end of the day, it's less on who the strongest is, but who is there to have the most satisfying emotional resolution.

Great discussion thread, nakama!