Forgetting about any particularly player, the hypothetical best player in my opinion must not have any weaknesses to pick on and as many offensive weapons as possible. Preferably the player should have an awesome serve, backhand, forehand and volley plus great movement. Let's say a player has most of this but he really has a poor volley. A guy like Djokovic may be able to somehow stay with this player and draw him into the net somehow and he would lose, despite being superior to Djokovic in all other aspects.
A player may have a problem with his forehand when players hit off pace to him. That's a weakness. Don Budge developed a weakness with his overhead so Bobby Riggs lobbed him to death and beat him in a tour because of this.
I've seen guys like Arthur Ashe go into the zone with huge serves, volleys, backhands and forehands, all strong attacking weapons. There is nothing to really work on. But Ashe often didn't keep it up and guys like Newcombe would wait for the letdown and often win.
So who in tennis history fits this? Remember the ideal is the fewest flaws and many attacking weapons.
I have my opinions on past players but I'll leave the present players out.