Ashita no Joe isn't a show about boxing, it's a show about a man who boxes. And yet it's the best sport anime I've ever seen.
But Ashita no Joe is less about boxing as a sport, and more about how it can become the very lifeblood for a man, and how that affects him and those around him.
The story of Ashita no Joe, seen through Joe Yabuki (amazing protagonist) as a character, was very influential to the young crowds at the time. Joe's journey through life, his strive to find meaning in a beforehand meaningless existence, his brash attitude, combined with the state of the Doya slums, created a very relateable figure for the younger crowd back in 1968.
It even went so far that they constructed an (real life) funeral for a deceased, fictional character. The creators themselves had difficulty believing how much of an impact Ashita no Joe had as a story.
While Hajime no Ippo (another boxing anime) had a huge impact in terms of Manga field because of its longevity, characters and other things, Ashita no Joe was a social piece of art which became the symbol of a young population protesting on the streets, it went beyond the borders of its media to become a cultural and social symbol.
Ashita no Joe is one of the few shows I consider peak fiction and Joe was the best main character I've encountered.