MotorTrend just did a comparison of the Big German 3 flagship sedans for their August 2009 issue:
1st place: 2009 Mercedes-Benz S550
"The Mercedes-Benz S550 does what a premium luxury sedan should: It makes you feel like a king.
And as a getaway vehicle, we can think of no finer way to escape."
2nd place: 2009 Audi A8L 4.2
"This is a distinctive, flavorful automobile oozing luxury feel. Add in its fine road manners, the stability of quattro all-wheel drive, and a "value" price, and the A8L is an easy recommendation for anyone in search of a great getaway."
"The gap from third to second is huge; then second and first places hang tight. Winner in its last comparison test five years ago, the A8L remains a delicious and compelling automobile."
3rd place: 2009 BMW 750Li
"No question the BMW is a fast, fast getaway sedan -- the one you want when you're carting a boatload of dough out of Terry Benedict's casino. Squash the gas pedal and the big 7 just goes and goes. The problem is, that's about all it does. Its lazy throttle response, turbo lag -- whatever the problem is -- kneecaps the car in situations when full throttle isn't required (99 percent of the time). I can forgive a luxury sedan that gets this formula wrong the other way -- buttery smooth most of the time, hopeless in full-throttle situations. This is unconscionable."
In summary: The Mercedes-Benz S550 won due to its all-around brilliance. Ride quality was exceptional, balance was excellent, skidpad grip was enormous, and acceleration, while not the fastest, was certainly a lot smoother than the BMW 750Li's.
It should be very interesting to see if the upcoming new Audi A8 can take the crown away from the benchmark Mercedes-Benz S550.