Board Thread:Naruto Discussions/@comment-25531508-20140923043228/@comment-25382671-20140924231727

Since we don't have access to his abilities when he was on his prime, I'll respect your choice.

Since it is a choice to think like that, since Kishi stated (repeatedly) in the Databooks that Hiruzen was "the strongest shinobi of all time".

I don't think you can really dispute with the 'opinion' of the author on things like that... but whatever suits you.