Can a defender be a 'best player in the world'? Watch
Thiago Silva is a great defender but does he have the talent of Lionel Messi?
I would say no.
I guess another way to say it would be to say that if the best attacker had skill level x at being an attacker, the best midfielder had skill level y at being a midfielder, and the best goalie had skill level z at being a goalie, then if the best defender had a skill level greater than either x, y or z, he'd be the best player (regardless of our lacking ability in measuring these variables).
However, the reason attackers win it more often is because they can change a game very easily by themselves. It's harder for defenders. For a defender to be considered great, he must have very-good defenders around him; or else he will be dragged out of position too much. Look at Thiago Silva in the World Cup. He was playing with a bunch of failed-wingers/midfielders. Therefore, it made him look average. Whereas Maldini & Cannavaro played with other good disciplined defenders, therefore they looked more solid. I'm not trying to say Thiago Silva is as good as them, but having good defenders around you is a must.