i think almost all sports have someone in the history who was ahead a the curve in ability. What you tend to see a lot is the best dont get worse its just everyone else catches up and thus the bar is raised. You also see points in history where two people come along at the same time and no one else can get close. If Federer and Nadal had careers at different times they would of both picked up a lot more titles.
Motorsport – Carlos Saintz, Shuey, Loeb, Rossi
Tennis – Borg, Federer, Nadal, McEnroe
Golf – Woods, Nicholaus
Snooker – Hendry, Davis,
Darts – Phil ‘The Power’ Taylor