I can think of two Aussie examples of a sportsman or woman who were the best in their sport at the time, and their results will never be bettered.
Heather Mackay was a squash player who never lost a match in her professional career (1962-77). She won the British Open (the defacto world championship) sixteen times in succession.
Don Bradman may not have had the most natural skills for a cricketer, but his batting is universally acknowledged as the best ever. By the most-used scale of comparison, batting average, he is 50% better than anyone else, and 150% better than most. A statistical analysis of the history of batting averages put his effort as 1-in-a-million, when there have been only 10,000-odd first class cricketers in its 150 year history.
Any others you can think of?