One of my friends feels that being a heavyweight champion in boxing is the best sporting achievement. He is an awful boxer himself, so he is coming from a biased perspective. However, I understand why he would say that. That is probably the most bravado accomplishment knowing, there isn't a man alive who can fight better than you.
However, I look at the best sporting achievement from a different angle. Some people have never had a fight or even thrown a punch. Boxing is a unique sport, and a lot of people have never participated. Because of this, I feel the best sporting achievement is being the fastest sprinter in the world. Specifically, all those gold medals and world records that Usain Bolt holds. Every single person, who is able-bodied has had a foot race in some way. All children have had a short foot race. The pool of people who have done this event is far greater than the amount who have ever fought or took part in boxing.
It's tricky for me to explain but hopefully, you understand what I'm trying to say.