The answer is easy: the one who's the most succesful player.
Unless you offer me your definition of 'competitor'
To me the BEST COMPETITOR is the one who NEVER GIVES UP, even when he's down 2-0 sets down, and finds a way to win when things get tough (i.e when their natural talent is not there on the day and they are forced to compete).
In other words, the best competitor is NOT THE ONE THAT PLAYS MAGNIFICENT GAME AND BEATS EVERY OPPONENT EASILY. That just means that he's much better tennis player than the opponent. But when someone is having a bad day, is down, BUT STILL finds a way to win, now that's a good competitor.