IMO, the factory designed/tuned DRC was hands down the best feeling suspension for this car from both spring rate and dampening. It came with a couple of design flaws however that has seen many abandon it to use a coil over or similar in it's place (Gen 1 seal design, and braided/rubber encased hoses that crack, rust, rupture. I have ridden in 5 different RS6's, and owned three all at one time - each with a different suspension set-up. My observation on quality of ride & performance from best downwards is as follows:

DRC
KW3
H&R
PSS9
Koni Yellow

I also ran a set of ST for a few months and abandoned that set-up as it had only compression dampening - no rebound dampening.... It would fall into the 6th place.