User blog comment:Wrath022/Chapter 524: Black Future, Review/@comment-26884814-20170226181338/@comment-7418318-20170227215131

I'd wager that most people in the story either never knew that Acnologia was originally Human and of those who do, many will probably not consider him to be a wizard any longer... which isn't a bad argument. You could say that a Dragon isn't a wizard any longer, or that he has moved beyond the level of still being considered a mere wizard.

That and the chance that they may be mistaken. Fearsome as Acnologia is, Zeref is a far more well known name. Only few people outside the magic council seem to ever have heard the name Acnologia.

Thus I don't think that people calling Zeref the strongest wizard in the story carries too much weight.