I voted GS - but only purely because it's the IS' big brother.
In actual fact, they're all very good cars and I've been in all of them except for the A6. The Lexus is most likely to have the best reliability. The Merc is (arguably) the best in terms of passenger comfort. The BMW is the most sporty and probably the best driver's car. The Audi is underrated but still top notch.