Not because he's the most powerful, but because of his actions.
Manga spoilers ahead!
Summary:
- MHA is a story of Izuku Midoriya becoming the №1 hero.
- While he's technically not the strongest hero yet, his accomplishments make him more worthy of the title than Endeavor.
- He's the greatest hero in the eyes of the people.
The very start of My Hero Academia (Boku no Hero Academia) tells us that it's the story of Izuku Midoriya who eventually became the №1 hero. So far, most fans expect him to become that at the end of the series after defeating all his enemies.
However, some fans argue that he already is the greatest hero — maybe not in the official rankings, but certainly in the spirit of the term.
Technically, Endeavor Is the №1 Hero
Officially, the current №1 hero is still Endeavor — Shoto Todoroki's father who wields the power of fire. He essentially inherited that position, as he was №2 when the former №1, All Might, retired.
However, if we omit the technicalities, Deku is far more worthy of the №1 hero title than Endeavor. Even putting Endeavor's treatment of his own family aside (he got his redemption arc, after all), Deku's accomplishments at this point in the manga show that he should be considered the greatest hero, considering he saved Japan — and probably the entire world — more than once.
Deku Very Much Deserves That Title
Deku was the one who stopped Shigaraki from destroying Japan by erupting Mt. Fuji. He held him off until All For One arrived, which is a big deal — Shigaraki defeated Bakugo with ease, for example. Saving Japan should already give him the №1 title, but it's more than just that.
The entire point of Deku's character is that, despite the fact he fights villains, he also tries to sympathize with them. A hero is a hero because they protect others, and that can be shown in protecting the villains from themselves and their own tendencies as well.
Deku showed how he can sympathize with others with Gentle Criminal, among others. In the latest chapters, he managed to even reach out to Shigaraki's humane side, which many fans considered impossible.
Yes, many of Deku's accomplishments are not something he could have achieved alone. However, he doesn't need to be alone: his friends are his strength as well. In that way, he's a thematic contrast to Endeavor, who doesn't rely on others much.
He Might Not Be the Strongest Yet, But He's Still the Greatest
However, others argue that the №1 hero title is given to the strongest hero, not to the one with the most achievements. That may be true, but even in that case, Deku's the greatest hero. Not the unquestionable №1 in the rankings yet — he still needs to master his Quirk — but definitely the greatest hero in the eyes of both people in the story and readers.