The 3 most Important are Luffy Sanji and Zoro. You’re overthinking it. Whether they’re in battle or just in the ship those three are the main characters.
On the ship, however, Nami and Sanji take precedence over both Luffy and Zoro (unless the captain actually has something to say, which he usually doesn't), since sailing to the next island and surviving the journey (i.e. eating) are equally important.
Overall, and if Oda truly let their roles be important, I'd rank them as:
- Luffy (captain);
- Zoro (vice-captain)**;
- Nami & Sanji (navigation** & survival);
- Robin & Chopper (historical knowledge helping to the navigation & medical aid that doubles as survival);
- Franky (ship maintenance);
- Brook (morale);
Jimbei** and Usopp are their own tiers because:
1. Usopp has been pretty useless post-ts; as a lookout/sniper he should be indispensable when approaching other ships/land, but so far this has been inconsequential;
2. Jimbei should be as important as Nami (helmsman) and as Zoro/Sanji (old, experienced consigliere, able to advise Luffy on any matter) if only Oda had been bothered to show Jimbei in this capacity at least once.