I can't actually go past Enel. Sure we've seen some people take their devil fruit powers to something unexpected (Robin, Luffy, Moria) and there a lot people who have shown a high level of mastery over their devil fruit, yet we've seen very little of (3 Admirals, Sengoku, the Supernovas), but Enel is quite possibly the one person in One Piece that we've probably seen the full extent of his fruit's powers unleashed.
As a logia, the Goro Goro no Mi is naturally a powerful fruit able to control electricity, sure. But I've seen plenty of logias already, and none have had the imagination and control Enel has displayed. Not only able to alter the number of volts in his electrical charges, but also able to give them amazing shapes and forms which serve different purposes.
And even though probably being the most skilled elemental logia shown to date would probably be enough, Enel was able to take his powers to different levels. Gold alchemy, creating heat through his energy, powering an entire ship and even creating an enormous lightning cloud able to destroy an entire island of decent size.
Sure, I may rate Enel as nothing special physically and mentally, but of all the characters so far in the manga he has shown the greatest mastery of his devil fruit.
P.S. I don't get the Luffy votes. Sure, the gears are quite imaginative, but he doesn't so much "use" his devil fruit as much as it's just a part of who he is. Honestly speaking Gear 2 was the first time he really "used" his devil fruit powers, the rest was just manipulating the body given to him.
That said, could just be how I've interpreted the topic. You may argue that manipulating his given body is the "best use" of his devil fruit.
EDIT: Accidentally called Enel's fruit Kizarus, whoops (thanks undertoe)