Well, it's just like instead their own adventure on ONE island seperately. They have their own adventure seperately each one on one island. They have the same place to return and that is Sunny, so it would be okay. No matter how long it took, because they have to be together in New World right. I guess Oda-sensei want to prepare something for them before them go to New World, like power-up, making more allies, making more nakama ect.
If I set up the order which SH returns first based on their reasons would be kind of wrong, because they all want to return, so I would set up the order by the easier way to return based on their situations.
1. Franky because he can built the ship by himself. Well, it would depend how far from his island to Sunny's island. But with his supa and technique he could learn from the country of genius. I think he could have no problem.
2. Nami. Somehow I got a feeling that she could learn some wind technique and used that to return as fast as she can. Who do you think is the best navigator in OP world?
3. Sanji because he wants to get back ASAP, as long as he gets away from that island.
4. Usopp. Maybe because he's nearby.
5. Chopper. Maybe he could use the bird to get back. Because South Blue is farther than the others.
6. Robin. I still haven't fingure out her way out. But after she learn the truth about the brigde, she could use the ship and go back. I think she in the country near the Red Line and Grand Line. The ship she can only use is the marine ship.
7. Brook. Because he kind of unknown to the world. He has to save the country first and then have to find the ship (I don't think the poor country have the ship)
8. Zoro. Based on his lost nature if he goes alone, so he has to cooperate with Perona if he wants to return. And it took time you know. He also need to took time to rest for his injury.
9. Luffy. You know the reason what he is doing right now.
But I hope they could return to Sunny at the same time.