I think the conversation certainly has to start with Phelps. The medal count alone is incredible. As far as the individual difficulties each sport entail, it is fair to argue which is tougher, but the thing that almost certainly has to be similar is the absolute dedication, these performers have. They start so young, and work long hours to perfect their sport. My son went to school with Phelps, graduated in his class and he was on a modified school schedule even then.
Phelps records right now stand alone, but Biles, Bolt and Ledecky, could approach him.
It is fun to watch the best in the world compete in so many different sports.