The 3rd took out 2 hokages and almost defeated a sennin. If that isnt proof that he is (was, whatever) the strongest ninja shown to us, then I dont know what is. And the fact that he did that well past his peak is even more showing of superiority.

There could be stronger ninjas, (imo if not him then jiraiya or itachi) but none have displayed, in battle, strength comparable to the 3rd's.