Did you drive the old XF or the new one? I haven't drivven the new one yet, but have high hopes for it.
I did have the new A6 for a week recently, will post a full review soon. I really liked how it handled with the lighter 2.0T engine. It felt smoother and easier in typical driving than the others, yet not numb the way the Lexus is. Also a more open interior, partly thanks to a lower, less intrusive center console than the rear-drive cars.
You don't mention the Cadillac CTS. It has the sportiest handling in the class.
Heading in the other direction, the Hyundai Genesis is a much better car than many people realize. It feels larger and heavier than the Audi, though, so not terribly sporty. Just a really good car for the price.
I agree that the MB and Infiniti are dated.