The biggest individual rivalry in sport is....
NOT Federer vs Nadal!! It is Federer vs. Woods.
Both players have 14 GS majors, and both players will attempting to win their 15th GS at the end of June. Woods is trying to reach Jack Nicklaus's 18 majors and Federer has already reached Pete Sampras's 14 majors, and 18 majors aren't out of the question.
It is fascinating to see how both players are chasing history in their own sports, but it seems that they are both challenging and motivating each other everytime they play.