I still don't get the Luffy arguement:
Firstly, by saying that Luffy "best uses" his devil fruit, you're basically saying that he has mastered it and reached the limits of his creativity. That is: no gear 4, nothing further except some physical upgrades. That or you're saying that Luffy is already the greatest devil fruit user on the planet though he's nowhere near the top of the world?
Secondly, people talk about mastery and refer to the latest chapter showing Luffy's struggles in his standard gomu gomu no pistol. For all we know, when Buggy first got his fruit he could only split once, Kuma's paws only made a shockwave large enough to push a leaf and Iva-san could only enhance his sex drive. Just because we haven't got five flashbacks of all these people as kids doesn't mean that there was no practice required with any of them. Luffy has mastered his fruit just as much as most other people, and probably struggled just as much.
Finally, the arguement that Luffy is the only one using his fruit in an unexpected way. Should I refer you here to Enel's gold alchemy? Perhaps to Moria actually taking shadows and shoving them into zombie bodies? Was Robin meant to make wings from her hands? Heck, I could argue that Walpo (the ex-Drum king) was never meant to be a factory but just be able to eat anything.
The point is: We know how Luffy has grown. We look at him every chapter and go "Omg I never would have thought he could do that gear thing", but then when characters like Moria are introduced we go "Oh yeah that's a nice fruit" and then don't really think about what they might have had to go through to get that power. For all we know, the shadow shadow fruit was only meant to make a Doppelganger, but through imagination and training Moria was able to, not only turn it into so many more forms, but then take and control the shadows of others.
So yeah, the gears look amazing to us who have seen Luffy's original powers, but really he's the only devil fruit user (potentially alongside Robin) we've seen "grow" in his powers.
P.S. After being introduced to Aokoji, who then could have imagined he could freeze two tidal waves in a second from a distance? Before then we believed he had to touch things to freeze them. There's plenty of examples like this.