I don't think this is an "authority" crew. They listen to Luffy...but just barely. It's more a group of friends, it will never be "first mate tells others what to do and they follow orders". I think it's wrong to think of the crew in those terms. Zoro would be the guy, but only for the occasional "if you take him back I'm out of the crew, we have to respect the captain" talks. Other than that, their roles are what they are. Cook, helmsman, doctor, shipwright, navigator, archaeologist, musician, sniper/liar, captain...and that leaves Zoro with no real role, so he's almost by default first mate. But again, it's not a "real" position in the crew.
Back to Sanji and what Moe is saying above, while I'd love for Sanji to fight Queen or King, it would also be cool if CP0 was still on the scene and Lucci goes after Luffy, but then "the Hunter" makes his return saying he doesn't get a rematch. Lucci would still be strong as hell now, stronger than he was back then, but Sanji taking one of Luffy's former opponents would be a cool moment.