I would say looking at their characteristics, and remember that like bounties strength doesn't mean rank(its a huge influence but its not sole reason for ranks). Smoker said in Alabasta that "there are people much weaker than him at much higher positions".
1.Luffy would be- Vice Admiral(has his own forces/ crew/ship/ base/he wouldn't be stationary though). I see Luffy kinda like Vergo a higher level of VA but a little like smoker, in which he only owns his crew and doesn't own a naval base. Still wouldnt fall under any other VA imo, I see him being the top of the top for VAs position wise.
2.Zoro- Might be VA or Rear Admiral or even captain( again remember smoker said in Alabasta "that there are people who are much weaker than him at much higher positions". I see Zoro as the opposite, he would be stronger than his position since he wouldnt be too motivated to do the whole marine thing)
but i don't see him owning his own crew, he would be an underling for somebody ship(or a lone marine wolf). I dont see zoro wanting to be in a leadership position, he rather somebody else be his "captain".
3.Sanji I can definitely see him as a VA(more Vergo than smoker VA status though), he has the whole responsibility thing and is pretty motivated about things( not saying Zoro isnt but Sanji is imo more I guess motivated/ less lazy. I can see him owning his own crew or a whole naval base like Vergo also, maybe he would be the leader of the marine corps special forces of chefs/ the leader of the group naval special forces located at the marine headquarters. He would be the chef for the marines but I can see him holding a VA/ leadership position. I can see him leading other VA and giving orders to entire fleets, he would probably be stationed at the marine headquarters or his naval base. I see his as being in a stationary position.
4.Robin- I see Robin as CPO member, an assassin/specail forces for the marines. She most likely would hold i guess a special rank(cant remember if cp9 members had special ranks but I do see robin having a special rank/privileged.) Maybe an rear admiral inspector or something. I dont think shew will be the leader of this group.
5.Franky- I see Franky as a commodore and I see him owning his own crew. Though he definitely will still be somebody underling/ somewhat. I see him traveling with his own crew alone while taking orders probably(kinda like old smoker).
6.Brook- Also a commodore/special rank, I see him as part of the assassination/special group with Robin. I can see him as a underling for special forces group though.
7.Nami- Captain, I dont see her owning her own crew. She most likely would be the navigator to a huger marine naval vessel, in which a VA probably is captain.
8.Usopp- Commodore/special rank- I see him being a stationary commodore at the marine headquarters, and probably part of a special group of snipers called for assistance when anybody calls for it. I can see him being well regarded for assassination missions and other special missions. I can see him, robin, Brook and sanji working close together for special forces at the marine headquarters.
9.Chopper- high ranked marine doctor. Probably another special rank, or a captain. Also stationary at the marine headquarters.