In my honest opinion I see the Goro Goro no Mi devil fruit as the most powerful in the One Piece universe. The only one I can honestly see easily defeating this devil fruit power is no other than the Gomu Gomu no mi user (Luffy). All other contenders will have zero chance (even with Busoshoku Haki) against the millions of volts generated from the Goro Goro no Mi fruit. Here are 7 reasons I believe makes this fruit the most powerful (read until the end because this fruit has one massive flaw). Let’s start with the most important point.
1. Opponents will not be able to withstand the entirety of the fight
This is the main reason why I mentioned that opponents with Busoshoku Haki have no chance against this fruit. Is known that the more Haki is used, the faster it will deplete. If you combine that with having to receive constant million volts electric attacks, the Haki will surely deplete faster, if the opponent manages to keep standing. Another reason is because of the main drawback of Busoshoku Haki:
“When two opponents using Busoshoku Haki clash, the one with the stronger Busoshoku Haki is more likely to be victorious. This can make it difficult to overcome an opponent's Haki in a brute force battle.”
This means that if the Goro Goro no Mi user combines Busoshoku Haki with electric attacks the chances of the opponent to win are nearly zero. It doesn’t matter if their Haki is stronger, all the Goro Goro no Mi user needs to do is continue to attack with electricity and discharge it every time their opponent touch them.
2. Can move and attack at the speed of lightning
This fruit allows the user to move incredibly fast and even merge with solid matter for traveling purposes. If the fruit user were to be constantly on the move, merging with solid matter to perform flanking maneuvers, and at the same time attacking from both short and long range, the opponent will have a really tough fight in their hands (if they manage to withstand it. See point 1). This alone guarantees the Goro Goro no Mi user victory against any opponent (other than Luffy of course).
3. Can melt gold (hence other materials) and create weapons
Gold melting point is 1,948 °F, meanwhile a bolt of lightning can reach 53,540 °F. The Goro Goro no Mi user has more than enough of a heat source to put nearly anything in flames or melt it down, as long as is flammable or conducts electricity. From what we seen so far the gold was melted by touching it, so with that in mind the Goro Goro no Mi user will be even more deadly in close quarter combat.
Also the user will be able to create any type of melee weapons they want with gold, meaning they can be experts in several weapons, making it even more difficult to defeat them. Another thing is that with the gold weapon they will be able transmit electric discharges in every attack without their opponent ever touching their physical bodies.
4. Can transform into “giant” form (Amaru)
For anyone (other than Luffy) fighting against the Amaru transformation is basically a death sentence. No one will be able to even touch the Goro Goro no Mi user in this state since 200,000,000 volts of pure electricity are involved. How many attacks in this form will the opponent will be able to withstand and more importantly for how long?
5. Can listen to electrical sound waves
Not only is this user deadly in short and long range combat, it also has the ability to listen to sound waves that traverse the air. This allows to analyze their surrounding with options to attack with either stealth or from insanely far away. The Goro Goru no Mi user will even be able to determine battle movements of entire armies and attack/defend accordingly.
Now the one reason why the Goro Goru ni Mi fruit is not the strongest fruit is because of this guy
YES the actual Goro Goro no Mi user is the biggest problem this devil fruit has. Eneru is insane in so many levels is absurd. He sees himself as an invincible divine god, ruler of everything, and because of this he doesn’t even care about his battles, hardly being serious, mostly absorbing attacks and humiliating weaker targets. But when he does not prevail (as it happened with Luffy) he gets completely overwhelmed by fear, cowardliness and in the case of Luffy he started to avoid him to keep his delusional “invincible” status he self proclaimed as having. Also his lifestyle is a huge problem, he spent it sleeping or eating in his residence while attending his so called “godly duties”. Of course he will see himself as a god of all, he his governing weak people with no challenge or immediate threat. He also doesn’t care about anyone other than himself and has no pity against his own subordinates. His only ambition is being the ruler of everything, returning to where god should be (apparently the moon). Because of these reasons the Goro Goro no Mi is completely wasted in Eneru.
That’s why I didn’t mention Eneru even once in the 7 reasons why the Goro Goro no Mi is the most powerful devil fruit. If someone else were to have that devil fruit and that person had a strong conviction to help others, defend their comrades with her life and have a very friendly/social nature (no god-complex whatsoever), she will be completely unstoppable. No one will be able to beat her since she will not fight just for herself. She will have a bigger purpose to keep going and being friendly will be a massive plus.
To sum up the Goru Goru no Mi devil fruit is in my opinion the most powerful fruit, but is completely wasted in Eneru. The fruit fit’s the character very well in the One Piece universe, but I can see him being defeated by many tough contenders no problem, even when he has the most powerful devil fruit and Mantra.