20 years ago I would've said: Mercedes-Benz, no contest. Easy win.
The quality and finish were without rival, safety was not even being considered by the other carmakers, the design among the purest ever with the brilliant Bracq then Sacco, the brand recognition didn't suffer any serious competition whatsoever, the engines were nothing short of brilliant.
Now it's considerably less obvious. I do not think any carmaker is able to truly distantiate itself from the others.
I'll vote Mercedes-Benz because they're close from my values, and because their latest models are promising in terms of quality. And they're still at the very top of the automotive world... but not really alone there any more.
The S-Class and crazy 65AMG do the job of keeping BMW, Audi and BMW at a distance, so I think MB is still ruling. Competition's getting close though.
Rolls, Bentley, Ferrari and co are too exclusive.