The only rule is not to use the Devil Fruits of other Straw Hat Pirates members. So in my opinion:
- Zoro: Neko Neko no Mi, Tiger/Panther model. The only fruit we see for Zoro is a carnivorous Zoan, only to improve his physical strength without changing his fighting style. A feline would be perfect for him.
- Nami: Goro Goro no Mi or Soru Soru no Mi. Nami already uses lightning and lightning a lot, maybe it’s too powerful for a weak trio member but we think she would be able to get the most out of the Goro Goro no Mi. The other option is the Soru Soru no Mi from Big Mom, Nami already uses Zeus but being the Devil Fruit user, she would create her own Zeus/Prometheus and be able to use them much more effectively.
- Usopp: Tori Tori no Mi, Model Hawk or Giro Giro no Mi. Hawks are famous for their eyesight, and being able to fly might give them an advantage. With the other fruit (Giro Giro no Mi), Usopp would gain incredible eyesight, along with X-ray vision. He would also be able to read information on the pitch. He would also be able to read minds, making his lies even more effective and exposing those of others.
- Sanji: Hi Hi no Mi. This fruit allows the user to freely create and control ice and cold temperatures. This would be an invaluable skill for a chef. Many recipes require the ingredients to be chilled, and having that ability handy would be a huge boon to Sanji. It would also give him incredible versatility in combat, as he could use ice as well as fire from his Demon Leg.
- Franky: Jiki Jiki no Mi by Eustass Kid. Franky is a cyborg, so it would be cool if he could be able to use a cheap version of General Franky (which can’t be used every time) by attracting metals. Plus, with his shipbuilding abilities, we’re sure he’d be able to find some creative uses. He might also have the Kira Kira no Mi.
We don’t want to give Jinbe one because as a fish-man he would probably get weaker and I try to keep the original fighting styles.
We didn’t use some fruits like Supa Supa no Mi for Zoro, Mato Mato no Mi for Usopp and Buki Buki no Mi for Franky because even though these Devil Fruits might match their fighting abilities, it won’t. not the same for their mentalities. We didn’t see Zoro using a Devil Fruit to create blades and fight with them instead of his own, Usopp using a cheat that would allow him to hit the enemy for sure without even an ounce of skill or Franky who would have basically the same problem as Zoro.