I'm making a few too many comments in this thread, so this will be my last. The issue is simply that he was introduced well before the ideas people are talking about were even thought up. The idea of haki didn't even exist and these guys had "mantra" which wasn't fleshed out at all and was absolutely what sparked the idea for observation haki. It was retconned to be that, but at the time the rules of it didn't apply in the way we know they do.

He changed his environment with his Logia fruit, something that we've only seen the admirals do, and MAYBE Crocodile (who was also introduced WAY too early and was shown to be much weaker than he would have to be in order to hold his position, and had weaknesses that made no sense as well.) if you count the sand storms that stopped only after he was defeated.

Given that change of environment, the sheer size and power behind his attacks, his speed because of lightning, and lightning being his power, he was absolutely incredibly strong. It was theorized by Oda himself that if he lived down on the blue sea that he would have a 500m bounty, an in reality he would know all about actual haki usage well before that point in time. He would have been a new world player without even a single doubt, and he would be extremely close to the abilities displayed by the elemental admirals.

He's a very hard character to place, because of when he was introduced, his ridiculous vulnerability, the fact that haki didn't exist at all, but at the same time the level of his attacks, his range and influence, were just, absolutely incredible. Definitely seemingly on par with the range and influence that the admirals showed.