In the real world it would be Chopper since the udea of making an universal oanacea is laughable but in OP world and with plot power he will do it by the time Luffy becomes pirate king or a little later at best.
Same for Jinbe, he is pretty much fighting vs the human condition pretty much: racism, elitarism, misoginism, hate vs minorities in general. It is in the human primordial and "animal" nature to try to dominate who is weaker or at least seems so. By in the OP world with the help of plot power it will be done by eos.
Nami, in a water world it would take an age but still it is a task that given a certain amount of time it is possible to achieve. Yet I'm sure that even if we haven't hear anything from Nami about her dream or her srawing a map or anything related to it since... Arlong saga(?) she will magically achieve it by eos or a bit later.
Sanji will get his all blue by Luffy destroing the read line linking all the seas.
Usopp pretty much has already achieved it.
Robin will have it done by eos, SH will magically get all the poneglyphs for her to read somewhat.
Franky will have it done by the time they reach Raftel.
Zoro will achieve it by beating Mihawk. This at least will require effort on panel.
And finally Luffy has the hardest dream cause since chapter 1 we have prerry much him and all the crew doing the impossible to achieve it. So in the real world Chopper at least will have an harder dream than Luffy (cause his dream is impossible, probably it will be easier to kick a ball from earth to the sun using just your legs than making an actual panacea) but since this is the OP world Luffy takes the cake even this time.