UK - a close third to Munich and Stuttgart, in my view
Good build, good quality and appealing (successfully) to the person who does not want to drive a Beemer or Merc but wants the "hard" facts of those two brands - quality, luxury, reliability, residual values and the like, but "understated". In the UK, they have been on a serious up since the mid-80s and the Quattro.
I had an Audi 80 for a while and it "did exactly what it said on the tin". Perfectly good, if uninspiring car.
The early versions of the A8 (I read somewhere) were all built at a huge loss as the VAG boss wanted a car, at any expense, to compete with the S class and 7 series. Totally over engineered.
The downside is (for many, myself included) the difficulty really telling the Audi A3 from the Golf, from the SEAT, from the Skoda. Similarly with the A4/Passatt. IMO, there's too many variants on the same theme which means that you look at an Audi and think "baby beemer" and not realising that the car is probably the equivalent of the M3.