Let's analyze everything...
Only for that is very hard to say who was more important. I'd like to analyze by replacing them about less skilled versions of them. Let's say a less skilled Nami, Sanji and Zoro.
A less skilled Nami may still not be enough to survive Reverse Mountain but if it does I think the crew would do just the same to the rest of their trip.
A less skilled Sanji would be enough to defeat Kuroobi and most Sanji enemies (including surviving Gin) but may not be enough to pull out Mr.Prince moments and the crew would be doomed in... Alabasta? Enies Lobby?
A less skilled Zoro would definitely not kill Mihawk's boredom and they are not surviving after Baratie.
So Zoro is definitely the most important and Sanji potentially more important than Nami but could be otherwise too.
At Shells Town we'd just have Luffy defeating Morgan and going to Orange Town next. At Orange Town I believe Zoro saves Luffy so the story would end here unless Luffy awakens
At Cocoyasi Village Luffy most likely would still connect to the people and save them from fishmen. They'd have no navigator and would sink their ship on their way to Reverse Mountain and the story would end there and no awakening would save Luffy. That considering that Luffy would still beat Buggy at Orange Town without Nami trapping his parts.
The crew would still cook and eat on their own but right at Cocoyasi Village we have Sanji fighting Kuroobi and saving Luffy. Without him one of the others would have to fight two fishmen (not together since Arlong was waiting for the fights to be over). Zoro wouldn't have a problem and Usopp ran away so much he wouldn't be a target. But without saving Luffy no one would stop Arlong so the story ends here
Only for that is very hard to say who was more important. I'd like to analyze by replacing them about less skilled versions of them. Let's say a less skilled Nami, Sanji and Zoro.
A less skilled Nami may still not be enough to survive Reverse Mountain but if it does I think the crew would do just the same to the rest of their trip.
A less skilled Sanji would be enough to defeat Kuroobi and most Sanji enemies (including surviving Gin) but may not be enough to pull out Mr.Prince moments and the crew would be doomed in... Alabasta? Enies Lobby?
A less skilled Zoro would definitely not kill Mihawk's boredom and they are not surviving after Baratie.
So Zoro is definitely the most important and Sanji potentially more important than Nami but could be otherwise too.