I guess Sampras since he faced more and better surface specialists, especialy on grass. Sampras's competition for YE#1s was pathetic though, and good to Federer for ending so many years at #1 with far tougher competition for that particular honor.
Neither would even approach Laver, Borg, Nadal (who had to face both peak Federer and peak Djokovic for years during his own prime), McEnroe, Lendl, Connors, Rosewall, and some others in the competition department.
