BMW is faster and is the fastest diesel saloon.
But it's not the ultimate. Audi A8 4.2 TDi is also fast, but it's a bigger, more luxurious car and, all things considered, is just a better car.
If BMW put a 380 hp diesel in 7 series, then you could make the argument that that's the ultimate diesel saloon.
I would still prefer A8 though :D At least from what I have red and seen on the internet. If i was really about to buy either of them, I would base my decision on first hand experience/test drives of both.