Defining what represents 'the best car in the world' tends to be something of a fool's errand. For as long as I can remember Mercedes-Benz laid claim to this strictly unofficial crown with their S-Class but they're German and they would, wouldn't they? Where did the corporate hype end and something approaching reality begin? Charged with establishing this essential truth, it was time to get on the horn to Mercedes and get some seat time in their S320 CDI.

Although it might seem a little odd to opt for this entry-level diesel version instead of a behemoth S-Class with more horsepower than a Saturn V rocket, it's exactly because of this car's relative affordability and relevance that the S320 CDI has a bigger claim on being the most representative S-Class model in the whole line up. That's not to say you'll be able to afford one by a sly bit of digging down the back of the couch. Prices kick off at £54,975 for the standard length car or, if you're used to turning left when you enter a 747, £58,975 for the long wheelbase model.

First impressions are promising. The car looks suitably imperious, especially when finished in black. Even the standard length car is a sizeable chunk of Swabian real estate, measuring 5,076mm. Opt for the long wheelbase car and the length stretches to 5,206mm, so you may need to take a tape measure to your existing garage. Width goes up by 16mm to 1,871mm and the S-Class also grows 29mm taller to 1,473mm. This is a car with serious on road presence.

Rest assured that this 235bhp common-rail diesel powerplant has the torque to shift all that metal at a decent lick. The sprint to 60mph is dispatched in 8.2 seconds and the top speed of 146mph isn't going to result in too many blown appointments, even if your commute takes you to the deserted autobahn between Stuttgart and Singen in the vanishing hours. The 368lb/ft of torque on tap is more than the S500 model can manage and means that the S320 CDI can change pace much like a white shark homing in on prey. The reward for choosing the diesel car over the S500 is a fuel economy figure of 35.8 as opposed to 24.8 mpg. Emissions of just 209g/km will make this car a tempting target for savvy corporate customers too.

"The one standard feature the S320 CDI seems to lack is any sort of Achilles heel"

What's more, because of the manageable ongoing running costs, the S320 CDI chalks up a comparatively reasonable 113p cost per mile figure, better than its diesel-engined rivals from BMW and Audi. In fact, that sort of running cost is about on par with far cheaper cars. Even a Land Rover Discovery HSE diesel will cost more to run. The engine is one of the best in the entire Mercedes-Benz family and although the S-Class is about as big a vehicle as you'd want to put it in, it never feels strained or overly vocal.

The styling takes a little getting used to, observers like me having grown accustomed to the sleek coupe-like lines of the 'old' W220 generation S-Class. The latest car smoothes the flanks with the absence of side rubbing strips but adds sharply-defined wheelarches that some commentators claim are reminiscent of the Mazda RX-8, along with a swage line that runs from the front wheelarch back to the tail light clusters. Bigger in every dimension to its forebear, this S-Class also features a bootlid that's raised. Mercedes claim this mirrors a styling cue from Maybach - their super-luxury vehicle division - but most of the car-buying public will more readily associate it with a BMW 7 Series, which isn't great news when trying to position the S-Class as a trend setter rather than follower.

That suspicion is reinforced in the cabin where a number of fascia buttons have been dispensed with, replaced instead by the COMAND (Cockpit Management and Navigation Display) controller, a rotary knob located between the front seats. Stop me if you've heard this one before, but doesn't that sound a great deal like BMW's almost universally hated iDrive or Audi's rather more acceptable MMI interface? Even the smooth expanses of dashboard and hooded instrument binnacle seem distinctly redolent of the BMW. Perhaps this is a little harsh. If Mercedes have judged this layout and interface to work adequately, pragmatism should dictate that they adopt it, rather than wilfully and expensively pursuing a design dead end.

Mercedes' approach in past times with its flagship saloon was to throw a multitude of high-technology features at the design, then wait to see what would stick (a theme continued by BMW's current 7 Series). More recently with the post-'99 version, Stuttgart dropped the gratuitous gadget count in favour of well-judged refinements - and this continues here. The problematic Sensotronic braking system of the E-Class has been ditched in favour of a refinement of the existing hydraulic set-up, dubbed Adaptive Brake. This system incorporates a number of safety features such as the pads lightly skimming the discs during wet conditions and a hill hold function. The foot-operated parking brake, never a hugely popular Benz staple, has also been quietly ditched in favour of an electronic handbrake. The S320 CDI features 17-inch alloys, seven-speed 7G-TRONIC automatic transmission with Speedtronic speed limiter and cruise control, adaptive brake lights (which flash in emergency braking conditions) and adaptive brakes that hold the car when stationary. These brakes are also fitted with ABS and BAS Brake Assist, while the ESP electronic stability program and ASR acceleration skid control keep things on an even keel. Metallic paint is standard, and nano-particle clearcoat will ensure a showroom shine for years to come.

The S320 CDI makes a compelling case for being the best luxury saloon around. The best car in the world? Right now it's hard to se anything else with a more rounded portfolio of talents.