I don't think that you can write off Robin so quickly. The only person to really beat her was Crocodile.she has already shown that she can slap away Zoro's swords, and she has beaten someone physically strong in Skypeia. Plus we haven't really seen her in a fight like we have with Zoro. Zoro's fights are always drawn out with him taking a lot of damage, and showing his abilities to the furthest. I think that Robin has more unshown potential.