But there is also a simpler way to distinguish between them, and one that takes far less account of such a rigid dynamic hierarchy, between those which are good, and those which are not.
It's not long since everything M Division touched turned to dynamic gold, but in recent years things have got very hit and miss, with success and failure at all levels of the ladder. So while some are still sell-your-kidneys hits - the M135/ M140i and M2 stand out as recent highlights - there have been some really damp squibs, too. Speaking personally, I don't like the overly aggressive M3 and M4 as much as any of the obvious alternatives, the last M5 didn't even set my trousers smouldering, and I'd question the very right of the X5M and X6M to even exist.
Enter the BMW M550i, to see if it can pull the sword from the stone of M's core values. As its name suggests it is one of the M-Performance models, sitting below the new M5 in the 5-Series range. But BMW GB has already confirmed that it won't be coming to Blighty, reckoning that the marketing gap between the 540i that tops the regular range and the M5 itself it too small to squeeze it into. For similar reasons, we won't be getting the diesel-powered M550d either.
The M550i's little brother status is similar to that of the M240i to the M2, with less firepower and fewer trick parts. Both M-badged 5-Series have similar engines, 4.4-litre twin-turbo V8s. The M550i boasts a peak 456hp, still a respectable tally but a way off the 600hp of the M5. The xDrive all-wheel drive system is described as being rear-biased, but it lacks the ability to go rear-only that the M5's big skid mode has. It's certainly not slow, though - on BMW's numbers it dispatches the 0-62mph dash in just 4.0-seconds, making it quicker than the last F10 M5 despite a 100hp power deficit. That's the advantage of all-wheel drive for you.
Let's start with what's good, with the engine at the top of the list. The big V8 is brawny and characterful, deep lunged and supplying peak torque from a diesel-rivalling 1,800rpm, but still happy to explore the upper reaches of its rev range. Under gentle use there's little noise beyond a muscular hum, but with whip applied the M550i moves with a serious purpose, pulling hard to the 7,000rpm redline and making some pleasantly angry noises in the top quarter of its rev range. It feels properly quick, not just tick-the-benchmark fast, with even a gently flexed toe creating a pleasingly forceful forward surge thanks to a pleasantly keen throttle response. Compared to the Mercedes E43, which packs a 400hp turbo V6, the BMW feels like it's brought a gun to a knife fight.
It also delivers all the other virtues of the current 5-Series. The cabin is well designed and well finished and packed with kit, including the semi-autonomous CoPilot active steering system on the car I drove. Materials are well chosen, the gentle LED lighting in the door trims is just so and the fonts and animations on the various display screens are crisply rendered. Cruising refinement is almost freakishly good, and the M550i is quieter over rough freeway surfaces than almost anything else. Even economy is respectable, with the trip computer's claimed 23mpg in U.S. gallons, on a run including both hard use and cruising, translating to 27.6mpg.
Have you spotted what's missing yet? Any hint that the M550i is anything more than a very fast, very refined 5-Series. There's a good reason for that; with the possible exception of the typing accident that was the X6M 50d I can't remember any car to wear an M badge while exhibiting fewer of the brand's key attributes.
The steering sums it up. On the regular 5-Series the reduction in steering feel to little more than digitally synthesised weight isn't really a huge problem, certainly not for the sort of typical duty cycle that a 520d driver is going to experience. While some genuine feedback would be nice, its absence is unlikely to be a deal breaker. But the M550i has what feels like exactly the same anaesthetised helm, finger-twirlingly light in Comfort mode and then with syrupy resistance added when switched to Sport. There's no issue with the speed of responses, it scythes into corners and the helm is deadly accurate. But it's completely lacking the chatter that used to make BMW steering feel so special, the lightening and tightening that indicated different surfaces and road textures. It's the M550i's considerable misfortune that the last M-badged car I drove before it was an E60 M5, a slower car that manages to feel about 600 percent more involving.
The rest of the M550i dynamics follow the same path of mild disinterest in anything other than covering ground quickly. The soft suspension struggles to maintain body control over rougher roads, even firmed up in Sport mode, and hard cornering reveals a surprising amount of body roll. It takes the combination of a slow turn and a big throttle opening to get the xDrive giving any hint of its rear bias, for the most part grip levels are high enough to make the 5-Series feel like a quattro-driven Audi. It's a neat trick, but not what most people are likely to expect from an M-badged BMW. There's absolutely no encouragement to go and play.
The debate over whether cars should be getting faster and more powerful is an interesting one, and an increasing number of manufacturers are using downsizing as an excuse to launch what are being pitched as back-to-basics products like the F-Type 2.0-litre. The M550i could have been something like that, a car that celebrated the virtues that made the less powerful M-cars of old feel truly special and which played the role of understudy to what will doubtless be the huge talent of the M5.
But it's not. It's nothing close. Dynamically it feels like a 530i with a warp drive attached rather than a true M Car, a very fast 5-Series for people who want to cruise in comfort and accelerate hard in straight lines. It's painful to admit, but we should be grateful that it's not coming here.
Engine: 4395cc V8, twin-turbocharged
Power (hp): 456@6,000rpm
Torque (lb ft): 480@1,800rpm
Top speed: 155mph (limited)
Price: €82,700 (Germany)