Having worked on all of these, MB, BMW, Audi -
I would have to say I am still partial to Audi.
In my opinion, BMW has declined drastically in both fit and finish, not to mention reliability. We see many N63 and N55 motors with under 30k miles needing turbos; injectors; timing chains; and plenty of bent valve jobs because BMW had the genius idea to NOT key the crankshaft! The new 3-series offering is complete junk with more plastic trim than a KIA. Does BMW even make a true M car - i.e., all individual throttle bodies any more?
And I would never ever consider a MB. Too much emphasis on gaudy curb appeal for the young generation and always good for an electrical nightmare.
If I leased cars, I guess my point of view might change.
Last I saw, the RS7 Performance edition was pretty impressive for its class.
https://www.youtube.com/watch?v=2gN6hFTCkEM