Let me name a few great atheletes, who have dominated their sport, like Roger Federer has dominated in Tennis.
Lance Armstrong *** (Should not be considered at this point. He has had impact for sure, just not the positive kind)
Well, you get the idea. I am talking of people who spring to your mind when you think of a particular sport. When you consider the following criteria:
(a) Performance in their main sport
(b) Giving back to the game (Publicity for the game, Player Associations etc.)
(c) Social responsibility
(d) Respect of peers
(e) Role model for youngsters
None from the above list makes the cut, unformly. For example, Jordan, for all his greatness as a player, is known as a difficult person to interact with. Michael Phelps is barely noticed outside of Olympics. I.e. He does not have much activities outside of swimming, even though he is absolutely stupendous in that area. Regarding Usain Bolt, similar story. Also, I think we need to wait a few more years to make sure that there are no doping controversies in the 2008-2012 timeframe (like what happened with Armstrong). Pele, Maradona, Tendulkar and Schumacher come close, but even an ardent sports fan like myself is not aware of too many activities outside of their prime activity (which is their sport). Also, in case of Maradona, I am not sure he is a great role model as a person.
Federer could go all downhill in the near future. He could get divorced and enter a marrying spree, get caught for doping, get caught for cheating and start dabbling with drugs. I dont know at this point and dont have enough powers to predict the future.
As of 12/18/2012 (Tuesday), I personally feel that Roger Federer is the sportsperson who has made the biggest impact in the history of sports.
Read this article by Lindsey Vonn for Sports Illustrated: http://sportsillustrated.cnn.com/2012/magazine/sportsman/12/02/roger-federer-lindsey-vonn/index.html?sct=tn_wr_a3