Nah I disagree with you there, Sanji always has a side mission. A rebel side that pays off at the end of an arc.
He sabotaged Enel's ship and saved Usopp and Nami, he became Mr. Prince in Alabasta to screw with Crocodile, He ran off to catch the sea train to save Usopp and Franky, He opened the Gates of justice in Enies Lobby. He ordered G5 around on Punk Hazard.
These are all things he just did without orders that were in the benefit of the crew. He thinks multiple steps ahead and has the added bonus that he is very strong and yes, very well dressed.
So if you can past the blood noses (last one on Fishman island i think but whatever) Sanji has more layers to his character than other strawhats. The current arc only adding to that in the form of backstory.
Chopper has been reduced to a mascot, and Law did the most crucial healing stuff most of the times he was around
Robin only appears if something needs further explanation or if a stone pops up
Brook, I don't know what Brook has been doing since he joined.
Zoro, always misses the middle of an arc, Oda just gives him a chore to pop up back at the end.
So that is what I mean with one-dimensional
Luffy, Usopp, Nami, Sanji (and Franky sometimes) do the more interesting stuff, interact with people, set things in motion.
That is why I feel that they are the more interesting characters.
But as you can see I did it well-argumented, You're free to disagree with me because it's my opinion. But don't be an asshat like some others in this threat who only judge by quirks because they don't comprehend what I'm saying.
You can laugh all you want, but it indicated only the start. The vinsmokes are just all and good introduced at this point. If you want to think Oda is going to wrap this up in 3 months I guess I will be the one laughing in the end. And deep down you know I'm right.
Hahahaha