The leading three of Germany’s luxury carmakers are Mercedes, Audi and BMW, and the competition between them remains intense. Until now that has meant who can boast the better performance, comfort and power. More recently, however, fuel efficiency has become a factor. These days, their diesel-powered executive sedans can match the fuel economy of a small car. The best performer on this front is Mercedes with its four-cylinder engine. But which of the trio delivers the most convincing all-round package?