I would definantly take the A6. I think (and so do most magazine's that test them) that the A6 has one of the nicest interiors in production. It's beautiful AND functional. AWD is a HUGE plus in the winters and that V8 is silky smooth.

The BMW and CTS will both be a little more rewarding when pushed hard (I've never actually driven a 5 series though, so I'm "magazine racing" on this one) but I think the Audi would be the best overall purchase you could make.