There is NO GREATEST SKILL ON THE PLANET! You could know a dozen tricks (like C. Ronaldo) and if you can't go past the defender, it's useless (again, like him too.)
There is no greatest skill, its being great at all of them to become the best player you can be.
FORÇA BARÇA!
"I’m a player who can do absolutely anything if I am smiling and laughing. If I’m not happy, then everything becomes more of a struggle,” explained Adriano.
Different skills are used for different situtation. If there was a skill that worked for everything every time, it would be the greatest. But since there isn't a skill like that, there is no greatest skill.
You could replace "greatest" with "most challenging", but there are already topics like that on here.