This was a no brainer for me, there is only one inspirational series that comes to mind for me when the word fighter comes to mind, Hajime no Ippo.
This series starts out as the classic underdog story that I personally love when done right. What do I mean about done right? Well I hate the kind of under dog story where the under dog through hard work always wins because they tried so hard. This is very unrealistic and though anime, movies and books make for a boring story. Yes these mediums are used as an escape for us the audience but for me at the very least they lose the entertainment value when the outcome becomes predictable when you know the hero will always win because of their hard work.
The best drama comes from watching a character learn and evolve from their failures and become stronger characters through their struggles. We get to see this in a character like Ippo who goes from a victim of bullying to a boxing star with the help of his friends. Along his journey to the top however he goes through so many ups and downs that leave the story unpredictable. Going into every boxing match you do not know if Ippo will win or lose the match and this adds an excitement to the series that one might get watching a real sporting event, well if your a fan of that sport. I must admit as much as I love watching sport anime I really hate watching real sport on tv.