All Might's Quirk — a mystery to everyone but Izuku — is called One For All, and it grants its wielder all the accumulated powers of every One For All holder that came before. As it turns out, All Might is much like Izuku. He was born Quirk-less, and only became Japan's Number One hero after inheriting One For All from the seventh user, Nana Shimura.
Here's an illustrative list of just a few of the amazing feats All Might has performed using his acquisitive Quirk: He lifted an entire sky bridge up to save the people trapped under it; He punched the 500-meter Tokyo Sky Egg up into the air; He once generated so much atmospheric pressure with a punch that it actually changed the weather over Japan; He blew up a helicopter in mid-air using his bare hands; He's also, of course, the only hero in Japan with the skill to stand-up to the Number One villain: All For One.
But what's anecdotal evidence worth without any hard data? Fortunately, the impact of All Might's work is also reflected in the numbers. Crime statistics show that crime dropped below 6% after All Might started patrolling the streets — and this was at a very dangerous time for Japan. Villains with powerful Quirks were running amok, and few heroes had the strength to bring them to justice. That's the kind of impact that any politician would be happy to run on.
Of course, careful viewers will point out that All Might currently suffers from a pretty serious injury sustained in his battle with All For One. That injury places a real limitation on the amount of time All Might can spend wielding the full strength of One For All, though we don't think this simple catch is enough to override all the evidence already cited.
All Might is without a doubt the most powerful teacher on My Hero Academia – at least until Izuku surpasses him.
