Luffy is far from the most creative user. He is creative, but he's lucky that rubber is so versatile and encompasses all types of techniques and potential abilities, this makes it easier for him to create new techniques.
True creativity, in my opinion, comes from a vast array of techniques from a devil fruit user with very dumbed down, invariable and specific fruit powers. This rules out Logia users altogether because their powers already have great articulation and versatility which is freely accessible to them. On the otherhand this also rules out Zoan users because their abilities are so limited that they lack the chance or opportunity to be creative with their fruits. There's not much you can do with just increased strength is there?
This in my opinion narrows it down to Paramecia users with very specific fruits. This therefore EXCLUDES the obvious choices; Luffy (Rubber), Magellan (Poison), Ivankov (Hormones), Hancock (Love), Pica (Rock), Daz (Blades), Moriah (Shadows), Kidd (Magnetism), Law (Modification), Shiki (Levitation), Isshou (Gravity), Kanjuro (Caligraphy)
The major contenders in my opinion have to have:
a) an array of techniques stemming from one simple power
b) techniques in all areas of use (offense, defense, supplement, other)
This narrows it down, in my opinion, to just three possible people
Kuma (Paws), Doflamingo (Strings), Newgate (Tremors)
They all show offensive and defensive techniques which are all very powerful and tricky to fight against. All being able to create mass destruction and harm single targets. And also for use of supplementary techniques, such as Kuma's "teleportation" or healing (reduction of pain), Doflamingo's doubles and "flight" and also Newgates landscape shaping (one could even go so far as to say terraforming)
So it's one of Newgate, Doffy and Kuma
I would say Luffy if it wasn't for the fact that Rubber gives luffy better opportunities to be creative than Paws, String and Tremors