Note: This was written keeping in mind FIFA 11, but it would work pretty well in any other version as well.
As a person who loves FIFA, I have played all versions of the game since the year 2000! Back then, all teams were more or less equal, and you could beat Brazil playing with DC United in the highest difficulty level! However, this has drastically changed over the years... In the latest version of FIFA, I find that the most difficult opponent is no doubt Barcelona (after the 'invincible' Classic XI and the World XI).
As a person who loves FIFA, I have played all versions of the game since the year 2000! Back then, all teams were more or less equal, and you could beat Brazil playing with DC United in the highest difficulty level! However, this has drastically changed over the years... In the latest version of FIFA, I find that the most difficult opponent is no doubt Barcelona (after the 'invincible' Classic XI and the World XI).