User blog comment:Wrath022/Chapter 524: Black Future, Review/@comment-26884814-20170226181338/@comment-7418318-20170227091104

And how do you judge who's out of who's league when both Zeref and Acnologia are basically immune to each other's attacks?

Acnologia because his being a Dragon protects him from almost anything that isn't Dragon Slaying Magic and Zeref because he literally can't die.

When it comes to raw magical power, I think Acnologia is very clearly superior, but when it comes to magical knowledge Zeref is the superior one.

So it really depends on how you define "strongest wizard". Is it the one with more raw power or the one with more spells, versatility and intelligence? If you think it's the former, then Acnologia, if you think it's the latter, then Zeref.