User blog comment:B214/Question and Answers 6/@comment-190.164.227.239-20161201033222/@comment-10330822-20161201211604

Well, he's still considered it canonically because nothing has been said about it since he was labelled the weakest. Ddraig said he was the "greatest", but that doesn't necessarily mean he's no longer the weakest, but the word "strongest" wasn't used.

Personally, I believe it has more to do with his reliance on the Boosted Gear. Instead of being Ultimate-Class with say just Scale Mail, he needs to use advanced forms like CCP and DxD that other users couldn't to defeat his enemies. You can't really say he's the strongest, if he couldn't beat a former Boosted Gear Scale Mail user while on the same level, using the same form of Scale Mail. But don't worry, I'm sure he'll eventually be laballed the strongest, his True Dragon body and devil reincarnation pretty much make that a certainty.

I don't think he's weaker than Vali at all, maybe physically, without armor. But they both defeated a Heavenly-Class Evil Dragon, even if Issei's form was nerfed. Apophis and Azi Dehaka were around the same strength-level, and Issei and Vali have very similar abilities. So, honestly, I would say they are at least equal, with Issei having the superior body of the two.