This may be a hot take, but I'd actually nominate Kaido's Azure Dragon fruit.
I know, it seems better to say the Nika fruit, but think about it this way:
Luffy took 12 years to master the basics of his fruit. Then about a few months to create Gear 2 and 3. Then two years to master those, and also create Gear Four. Then about three or more months to master G4, and finally awaken his fruit. All in all, it took 14 years and some months to master 3 of his gears, and the last one he still can't completely control yet. His time limit with it is kind of pitiful sometimes.
But that isn't a big issue with Kaido's fruit. Iirc, from the jump you can access your Hybrid form and Zoan form. Now obviously stuff like Kaen Daiko and other high quality moves will need lots of training for you to do, but you're still going to be pretty strong and capable regardless. It gives you heightened strength (to the point where you could beat your base self around a mid diff at most, probably), heightened durability, heightened speed, etc.
It's also quite durable. You have lightning screams, fire breath, flame clouds (that can lift AN ENTIRE ISLAND! DAMN), huge air slashes, and you can create big streams of wind, among other things.
It also grants you flight, of course.
It's overall just a very great fruit that has a lot of uses even outside of combat. But even strictly talking combat- You could probably win a war against a whole country in a few minutes with that fruit, as long as you're relatively skilled with it. It's busted as fuck.