I'm fan of complex design - design that makes you think. Simple design can be done by everyone, complex one is a work of genius. Conterporary premium brands have to show complex and dynamic design - to stand out from non-premium brands. More and more regular brands are using hi-tech features, powerful engines, quality material. So design is becoming more and more a factor of distinctness (between regular and premium brands).
Nowdays all the premium brands (BMW, MB, Audi, Lexus, Infiniti, Cadillac, Jaguar, etc) aims to be luxurious & sporty at the same time. And design is the best way to express sportiness and luxury. So ...

BMW - the best design for sporty premium brand: brave, complex, brlliant details, absolute dynamic.

MB - new models (SLK, CLS, ML, S, R) are showing MB design is heading towards more complex design, yet staying elegant. Amazing details.

Audi - too conservative (despite new grill); side panels are way too simple for sporty premium brand. Details are nice, but too simple. I hope Audi will excercise more brave design in the future (new TT, new A4 and A5 will reveal future Audi design). Their slogan is: "Vorsprung durch Technik" after all - so I hope their design will also follow that motto.