Well, I am a MB fan.

However, for me and from a theoretical PoV, BMW is undoubtedly the design leader of the three (even if I hate their look, it stands out and was an obvious inspiration especially for MB). Audi had a very pure design, but their strategy of copy-pasting the design with only one or two differences like the tailamps is showing a lack of new ideas. Usually one new idea every three or four car (new spaceframe grille. Then new interior. Then new rear design).

So I would say, the design leader is BMW, the usually more attractive car is MB, the most Germanic design is Audi.

All that seen with an "unbiased" eye, because personally I would rank MB first, Audi second, BMW third in terms of pleasantness of design.


Concerning the true classic, I think the most likely to get a "design icone" status in the history is the CLS. This will mark the history of MB, for me.
Another strong candidate is the e65 for its new ideas and unique design theme.
Concerning Audi I would say the first TT.