Most fierce competitor of all time
This is not about accomplishment/talent but strictly about a fierce competitor.
I would Nadal on the men side and Seles on the women side. Some of you might say Connors which is fine. McEnroe once said if he wants to increases the competitiveness level, he emulate Connors since he think no one wanted more than him.
Federer > Davydenko > Nadal > Federer
Nadalgaenger: "Djokovic is clearly better than Pete"