Since I player-lock on the GK, I guess the toughest one to beat would be myself (even when my stats are realistic becuase I am pretty damn quick over 20 yards). I once made a run up the field outside the box, made a rather poor pass, then rushed back to goal. Van Nistelrooy took a shot at me, and I made a no-look,overhand forehand save (like the ones in tennis, yeah). I even had to go as far as diving to the right to punch a shot clear after rushing back, TWICE in a game!
But for the default keepers, it has to be Dida. He's great when I control him, but when he's against me, I can rarely score on him.