Superb. The best in the world? There's a question. How can that be defined? There are definitely faster, more involving drives, but none that are as comfortable and refined. It's out on its own on as a world beater on those criteria. But we knew that already from the S 500 we tested last week, so it's the diesel engine under the S 350's bonnet that's up for judging here.
And it's business as usual for the engine; it certainly hasn't got any worse in its switch from old S-class to new. Acceleration is brisk, there's plenty of low-end torque to call on, for the most part it's inaudible from inside the cabin, and it is as buttery smooth and refined as the rest of the car.
Issues? There are a few, although none is a deal-breaker. Throttle response in the cosseting Comfort mode needs to be sharper, particularly when you need a sudden punch for overtaking but this is down to the seven-speed auto' gearbox (another carryover from old S-class to new) as much as anything.
There's also the slightest of gruff notes at start-up, a noise that reappears albeit louder under hard acceleration, something that should be a no-no in a luxury car. It's also not the most willing engine to be revved, but then your average S-class owner is hardly likely to give it the beans, or at least instruct their chauffeur to.
The rest of the dynamic package is typically excellent. The ride quality is superb, it corners flatly and handles with confidence-inspiring stability rather than any great level of driver involvement.
The ride could probably be made even better still if Mercedes' Magic Body Control (an incredible piece of engineering that can make speed bumps feel like they aren't even there) were to be offered, but it's only available on V8-equipped S-classes. Shame, but S-classes without it hardly feel like a poor relation.
The interior comfort is beyond reproach, the overall functionality and design less so. The twin front screens are a bit ugly, and the optional pair you can have in the back can only be operated by a fiddly remote or a smartphone app when you'd think they'd be touchscreen as standard in this iPad era. It can also be quite hard to find the button or menu for the command you actually want (skipping tracks on an album for instance).
Still, if all this winds you up you can always slip into the back seat, recline almost flat and have a hot stone-effect massage.