A good amount of characters in the series rely on Devil Fruit powers for fighting. Suppose that these characters do not have the fruits, do you think they'll still be as strong as they are currently?
Luffy should still be as strong as he is, as he trained off-screen during the 10 year time skip, but the way he fights probably won't be interesting without a rubber body and can't do Gear 2, and he will never win against Eneru either.
Crocodile and Eneru seem to rely mainly on their DF, but they must be physically fit at the same time to be able to take heavy hitting and yet still stand. Plus, they also have a secondary weapon (hook and trident) which they are shown to use rather effectively. I'm sure they can handle themselves in fights even without fruits, but they probably won't be feared as much.
Aokiji… He is considered the most powerful Marine because of his powerful fruit, and he seems to always rely on it in battle. We know that his bad point is being very lazy, so he probably won't be the strongest if he doesn't have the fruit.
That's all I'm saying at the moment. Discuss if you want to.