HANDS DOWN Ope Ope no Mi. It's far more versatile than almost any other of the fruits, save for the Gomu Gomu and arguably Ito Ito.You can use it for infiltration, sabotage, mass decimation of your foes and their foot soldiers, even practical things like surgery or moving through the air. Having essentially total control over everything around you is very hard to beat, and if not for the Deus Ex Eiichiro known as Haki, I'd call it just shy of unbeatable.
And, sorry for hijacking the conversation a bit, but after the TS, battles have come down to Haki as the main decider, not clever things like Luffy's rubber body making Gear Second or Gear Third possible, or elemental relationships. Haki itself is already pretty limited in creative use, because any variations aren't from using it differently, they're just from how strong you are in that type of Haki.
Point in case is Katakuri, as he's not really using his Kenbonshoku any differently, he only has more skill in it, meaning any character with it could hypothetically predict the near future. Busoshoku is essentially just more attack power, so everyone uses it to either punch, kick, or cut. Even Haoshoku is just "knock out enemies by the dozen". There's not much room for creative usage there.