Audi - nice balance between luxurious ride and driving fun, has quattro, and new Audis have stellar technology. It is also more reliable than the BMWs. I've had 4 Audis. The A6/S6 for a luxurious ride, and S5 for a sporty ride. If you can afford $110K, get the RS7.
BMW - a driver's car. you stick to the road and can feel the road. It's comfy, as long as it's not the sports suspension. Bimmers usually have electrical problems - that's what mine had. I've test driven the 4 series and love it. The 4 Series Grand Coupe has more luggage space due to its Liftback design. The M3 and M4 are pretty fast and can cause you whiplash
Mercedes - it's ok, but the controls are kinda old fashion. I rented one last summer to drive around Scotland and it performed well. The new Mercedes looks good outside, but I still prefer the interior of the Audis. For fast Benz, get the AMG version.