There's really not much between the three premium German manufacturers (i.e. BMW, Mercedes and Audi)... but the most popular is definitely BMW.
All started in the 1980's... when everyone was a money grabbing b@stard lol. If you were young and successful, then a BMW 3 series was THE car to be seen in. Back then, a Mercedes was seen as an "old mans" car, and an Audi was seen as you weren't as successful in life otherwise you'd have a BMW. However, in recent years, BMW have become too popular and I now see them as (dare I say it) a bit common (The traditional Mondeo man is now BMW man ffs)... In short, BMW has become a victim of its own success and allowed Audi and Mercedes to catch up.
If it's pedigree, then out of the two, it has to be Mercedes; as it's this manufacturer who patented what is widely accepted to be the first motor car in 1884. On the other hand, every Audi is basically the equivalent VW model, but in a smart business suit. The only exception here is the R8, which is basically a cheap Lamborghini Gallardo.
However, if it's true luxury you want, consider their bigger brothers
Audi= Bentley
BMW= Rolls Royce
Mercedes= Maybach