He supported Zoro of all people against her in EL, and him speaking up and agreeing with Zoro is basically what shut down all the others trying to say his demands from Usopp were unreasonable.
Supported another one of the main 3 characters's stance over her's.
Oda also literally named the half of the crew that included Nami and Sanji after Sanji and his trademark eyebrow shape, the way the whole crew is named after Luffy's Strawhat, having him as the temporary captain.
Being captain or not isn't the discussion right? Nami has been given more character moments and is generally treated like a MC as opposed to Sanji.
Luffy, Zoro and Nami were the initial 3 strawhats. Infact Nami was there on the cover of chapter 1 for some reason. This trio handled most of the decision making for the majority of the pre timeskip and the post timeskip. This trio or atleast 2 of this trio has been given more shine in post timeskip arcs where the crew was split up. Once again, even in the Curly haired pirates, Nami voiced the reason to go to Zou, Nami made the decision to help the Minks.
More recently the cover of Chapter 1000 had Luffy, Zoro and Nami - the three MCs. The iconic panel in Jaya where Blackbeard says "A man's dream never dies" had again this trio.
The recent eye catchers in the anime had this trio. They were the first three strawhats. There is infact lots of evidence as Morj pointed out in that video.