As I am somewhat biased I think the C5 RS6 is a very hard act to follow.
It sounds sounds lovely, it is very practical, it is quick, comfortable, and looks pretty good from many angles.
I of course loved my previous V6 Alfa164, Alfa 75 twin spark and one of my first cars was the lovely Alfasud Sprint Veloce.
The original Audi Quattro turbo was also an excellent car though I did want to swap it for a Lancia Delta Integrale but for all round practicality the RS6 is incredibly hard to beat (fuel consumption when having fun excepted).