I actually think that the 50% are some good value. Since Luffy always gets a huge power up during importan fights and he still got a lot fighting to do, before even facing the first Yonkou, his current strengh would still be huge, but leave a comfortable place for improvement.

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But I still think that Rubber + Pure Strenght is the key to deliver the strongest fists here.
I really like that concept of Devil Fruits. It shows the power balance throughout the storyline. At first Logias are the most powerful fruits in any possible case. Now, further into the story the balance shifts.
In the end i would bet on parameica fruits, that alter your physical attacks, to be the strongest. Excluding Earthquakes in this matter, Luffy, Bellamy, even Miss Doublefinger have some pretty potent abilities. In a 1v1 match-up everything that boosts your raw physical strengh counts most, if you have a strong Haki.

Zoans are pretty good to, but i would rather consider them midclass, since they do increase your power, but not in an unnatural way like parameicas do. The possible multiplier is not that high in my opinion.