Why number 10 in soccer is the best?

Answer 1

It isn't at least not by definition. It's worn by the attacking central midfielder, who's job it is to create opportunities and make plays. The number 10 player in a team might be the worst of them all, it depends on the player.

Answer 2

number 10 was the shirt number of Pele and maradona and therefore it signifies flair and class - the coach gives it to the best player [not always] as an award for his skills