If I were in the fortunate position to actually be able to purchase one of these vehicles, I would most definitely go for the A8. There's not that much to separate the bunch in an objective manner, so it's all down to subjective points such as design. On that point, the only one to truly appeal to me - the only one I would not change anything about even if I had the chance - is the Audi.
Here's my entirely subjective rating:
1. Audi
2. Mercedes
3. Jaguar (why, oh why did they have to give it that stupid
Cars grin?
)
4. Bentley
5. BMW
6. Lexus