Enel IMO is still the most powerful DF-user we have seen. Lightning in itself is one of the most powerful elements out there. And Robin said it when she encountered Enel that out of all DF's Goro Goro no Mi "Boasts to be Invincible"(or something like that). And Robin is one of the most knowledgeable characters we have seen or even heard of in OP. Only one with more knowledge about stuff like that, that we have heard of would be Dr.Vegapunk. Combine that with his Mantra, which his DF even enhanced to godlike proportions.
Enel's only weakness was that he had never fought anyone even remotely strong. He had no battle experience. And on top of that he was so arrogant because he thought he truly was invincible. So when he fought Luffy he was reduced to a panicking fool. Blackbeard said this when he fought Ace. When you remove a Logia-users invulnerability he usually becomes even weaker then a normal fighter. Ace was strong even before he had his DF. Enel wasn't. Just imagine how powerful Enel would have been a pirate and been a faced real enemy's even before he got his DF, like Ace. He would have been so overpowered he would probably have been considered even stronger then WB.
Kizaru strikes me as the same. Someone that doesn't have any real fighting power without his fruit. He just cruses around on his DF power. In theory his power should be almost as powerful as Lightning. But from what we have seen, when Kizaru are about to travel in his light-form, it takes him several seconds just to convert to light. When Enel moved in his form it was instant. No chargeup needed. Closest thing to teleportation we have seen. If this is because Kizaru is inexperienced with his powers because of his playfulness/laziness or if his power just is that much weaker and slower then lightning we might never know.
And for the difference between Kizaru and Smoker, is that Kizaru's Logia gives him so many properties, both offensive and defensive. Smoker didn't really gain anything more then the Logia invulnerability. He didn't really get any physical speed or strength from it. Smoke in itself isn't really a weapon. He can grab stuff with it, but that is about it we have seen so far. Light as we have seen has enhanced Kizaru's speed in his kicks. And as he pointed out, speed = power. He can also travel real fast. And light can be use as a weapon too when concentrated to a laser. Ice, Magma and Fire all have defensive and offensive properties that make them much more powerful then smoke. Sand too. Even though it doesn't really offer any defensive ability more then the standard, we saw Croc use both that draining and splitting waves. So even if Smoker learned how to use haki and learned how to use his DF better he would never have the same potential as the other Logia's.
Just imagine if you merged Rob Lucci and Enel together. Someone with Lucci's battle experience and fighting style, but with Enel's DF and his Mantra. He would be able to conquer the world by himself.
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