If Roger Federer keeps going the way he's going, he could one day be considered the greatest sportsman in history.
Update: Via email, a nomination for Pakistani squash player Jahangir Khan, who engineered a 5+ year unbeaten streak during which he won the International Squash Players Association Championship without losing a single point. (thx, abbas)
Update: Also via email, a vote for darts champion Phil Taylor, who has won 13 world titles, including 11 out of the last 13. (thx, krush)