Yeah the mantra is exactly what makes him shine amongst the other logia's. You could almost never dream to hit Eneru.
Lightning does indeed seem like the most powerful logia power to me (closely followed by ice/fire), but it's a bit hard to reason about it since Oda didn't handle it very realistically. Most notably the rubber-thing - as has been repeated ad nauseam, 100 million volts would most definately 'overcharge' any isolator and make it conduct, melting it instantly. Then there's also the fact that he never managed to kill anyone, which is because of the 'I don't want anyone to die' policy Oda holds. Mind you, I'm not complaining, story-wise Oda handled it very well ofcourse, just was some "pseudo-realism".
A thing that did bother me a bit more however, was that Luffy never actually should have been able to defeat Eneru..like I said somewhere else already, Eneru's mantra somehow "failed" to work on that last hit. Luffy charged from miles away, and all Eneru did was mutter "it's fast".. I'm sorry but if you can look 2 seconds into the future or whatever that mantra thing did, then it doesn't matter if the attack is fast, you can dodge it. Again, it was the story that made this 'glitch' happen.. it woulda been kinda anti-climactic had Eneru dodged Luffy's golden ball attack (like he should have): Luffy rings the bell. Everyone happy. Eneru owns Luffy. Eneru starts his Advent of Thunder and destroys Skiepiea. Yeah..
So in conclusion, at face value Eneru is definately one of the most powerful foes - and CP9 is more of a downgrade in enemy strength. One of the most powerful elements as his logia and his mantra.. this kind of strength would be expected of Whitebeard lol.
The only person I could see beating him, from the ones who we know what their powers are, is Ao Kiji. Ao Kiji seemed to me to have enormous control over his Logia (more so than Eneru), and according to what Crocodile said, this is what really makes the difference.
(I actually wanted to make a thread like this a while back, but I forgot lol..)