Not against the Audi A6 
5 Series is a contender 
Considering a newer Mazda 6 SUV test Drive.
Another mate swears by his Kia Sportarge (His other car was a E350 just changed it for a C Class I think) but prefers the Sportarge.
Also considering Volvo 
………
I think the major consideration for us is the comfort in case we get stuck in traffic for hours. 
Audi A6 is a big, quiet comfy car (I have the Avant version) but I’ve also had plenty of BMW’s of various models. I have no real issues with my A6, it’s very quiet inside the cabin due to acoustic glass (it’s double glazed) but from personal experience I can say that the bmw infotainment system/Nav is leagues ahead in both speed and user interface.

Also, up until recently the bmw gets the 8 speed ZF auto box, which is a great combination, whereas only the most powerful Audi’s get that box, RS6, BiTdi’s etc. The others get the S Tronic, which is generally fine but it’s not a patch on the ZF8, especially in slow moving traffic. The average Joe wouldn’t notice though.

Up until a few years back, a friend ran a chauffeur business. He ran a bmw 730dL (long wheel base) and a Lexus 600h. He ran the bmw for 465K miles (gearbox overhauled at 265K) until he retired. The Lexus was a strange choice. It had massage function, a fridge with champagne bottles and glasses, rising foot rests and reclining rear seats. It ran on electric in town but when out of town the 5.0 V8 fired up.