Nico Robin has the potential to be the strongest of the Straw Hats because of her Devil Fruit
I want to say this right from the get go: Nico Robin will never be someone like Luffy, Zoro, Sanji, Chopper, Franky, or Jinbe in terms of raw, physical strength and fighting skill. But I believe that with the power of her Devil Fruit, she has the potential to be the strongest of all of them.
When you consider some of the things she has done with it pre and post-time skip, and the potential that would probably never happen, the Hana Hana no Mi is incredibly powerful. Apart from the usual Devil Fruit weaknesses, the biggest drawback of this ability is that it’s only as strong and as durable as the original person. And Robin isn’t that strong or that durable. So for the purposes of this argument, instead of imagining Robin with the power of the Hana Hana no Mi, imagine Zoro with the power of the Hana Hana no Mi.
Example 1: Giant appendages – “Mil Fleur” (“Thousand Flower”) is the mass summoning of presumably a thousand limbs to create a single, giant limb. This has been shown pre-time skip as being strong enough to hold Oars arm behind his back, and post-to push Sunny out of danger while underwater, and several times to beat up large crowds of infantry. But again these are only as strong as Robin. Sure the quantity and mass itself is typically enough to make up for it, but again imagine if they were Zoro’s arms and not Robin’s arms. They would have tremendous strength. They could easily go blow for blow with something like Gear 3 or Gear 4. Not only that, an important thing to note here is that she has produced both giant arms and giant legs. There is no reason to believe that, with enough training and focus, that someone wouldn’t be able to summon a literal giant version of themselves. This is not that unfeasible when you consider that she has already shown she is capable of producing a clone of herself already (see Example 2). So imagine the potential to create, at will, a giant with the strength and durability of someone like Zoro.
Example 2: Multiple Bodies – “Cuerpo Fleur”(“Body Flower”) shows that the user of the Hana Hana no Mi has the potential to create something as complex as their own clone. While so far she has only shown she is able to make a single clone, again with training there is no reason to believe that she couldn’t create more. I’m not suggesting something like an army, but even something like ten would be more than devastating when you again consider they all have the strength of Zoro. Almost every person who has fought Zoro has struggled against him. Imagine if there were 9 more of him.
Example 3: Variety and Flexibility – Along with the 2 above examples, there have been a number of different techniques that Robin has used that highlight the utility that the devil fruit offers. She made: wings, nets, stilts, ladders.