I have been lucky enough to spend some time with an Audi A8 equipped with the Bang & Olufsen "advanced sound system", and it is by far the best audio system I've ever heard in a car, period . . . whether stock or aftermarket/custom.
I second that. Too bad is sooo expensive. The tweeter towers popping up from the dashboard are awesome.
The other excellent system I have heard is a Naim system in my neighbour's Bentley Continental GT Speed.
Mark Levinson systems that can be found in variuos Lexus models are all overrated IMO (heard one in LS460 and SC), although still better than bose from my Audi, whcih is a complete crap.
(I'm a former car audio "Pro 600" champion and GASCA - German Auto Sound Challenge Association judge).