I would have said Michael Phelps in his sport, but then again, he did lose the last couple of races.
Michael Jordan would be considered the best in his sport, but Wilt Chamberlain was also considered the best (100 points a game).
If everyone started at the same age, same circumstance, same rules, etc..., then we can tell who was the greatest. But with steroids, enhancing drugs, stronger and more physical players, it's tough.
I agree that it's hard to find anyone who is the greatest in their own sport.