I voted for Porsche, the designs of their cars is flawless(except for the Cayenne). IMO Audi, MB and BMW share second place. IMO non of they have flawless designed cars.

MB: The design of the latests MB cars are good but somehow confusing with MB taking elements of one car and putting in on another(i.e MM rear and SLR nose).

BMW: Some bimmers look fresh while others look to extravagant or weird IMO.

Audi: Their cars look good but they lack that "hotness"(i.e 911, CLS, Z4).