In the UK, the A6 came--at various stages--with the following engines and transmissions:
2.8 30v 4wd
4.2 4wd (badged S6 in the UK)
The V-6 engines are all very smooth and refined; definitely a match for the BMW in line 6. The 2.6 when mated to the 4 spd auto was perfectly fine.
The 4.2 is a good engine, too. A different car, I suspect, to the tame 2.6!
Audi produced the best diesel engines at this time. In particular the A6 2.5 V6 TDI diesel.
1999 SL 500
1993 320 TE
1986 500 SEC