Cant ser how OP have a deuteragonist like for example Sasuke in Naruto or Killua in HxH.
To me seems clear that Oda chose other format for the characters, Luffy is a stand alone main characters and the SH and other more relevant characters work as support characters for him, each for a specific part of Luffy character and in different order of importance.
I would put the order in this way:
-East Blue four: Its clear thay they have much more relevance in the story than the other SH, and each of them have a very specific role in relation to Luffy and the fact that they are all around the same age and started the jouney together in east blue is to give the vibe of a group of young friends setting to explore the world.
-Robin: Robin for sure have a special place in the story as she is a key characters for Luffy dream and is the characters Oda use to connect the SH to the WG and the world misterys, and because of that Oda have given her much more development than the other SH, even if she has been kind of sidelined after the TS.
-Law and Ace: Them are the main non-SH guys that set the story in motion in each of the parts of the manga, both having a strong connection to Luffy and to other things in the world, being the bridge between Luffy and those things, and Oda make both of them D. to futher increase the importante of them for the world events.
-Chopper: Before the TS he got a importance nearly as big as the EB4, but since then he have really become just the support character that heal the group whatever is needed, but with a very low story impact.
-Franky, Brook and Jinbei: All the three had theirs moments in the story, but today they are there just to make for the numbers in the crew and be used as plot tool when theirs specific function is needed, expecially Franky when something is need to be build or some plot connect to Sunny.