Of course it's useful to have reference points, and I've used Sabbath and Priest as the main kicking off points along with Maiden.
You say "most HM groups will probably quote Sabbath as their primary inspiration". This is not the approach I'm trying to take here - it's a speculative guess, and that's not what I'm trying to do here.
If I was to sepculate, I would wonder how much of that is true about the NWoBHM bands, many of whom were probably equally influenced by other bands, such as Deep Purple, Led Zeppelin, Hendrix and other proto metal bands.
Factually, but unsatisfyingly generally, the NWoBHM was a time of immense importance in the development of the metal sound we have now, and at the time that was Heavy Metal - and it's still called that, even though it sounds very different to Metal today.
The NWoBHM is very interesting because of the crossover between "Hard" or "Heavy" Rock and Metal. There was no actual borderline, yet there were distinct differences.
Sabbath and Priest were just part of the landscape of the NWoBHM along with fellow old-timers Motorhead, UFO, Rainbow, Whitesnake, Thin Lizzy et al - and all those bands trod the border between metal and hard rock.
The huge numbers of new bands and styles that sprang up during the NWoBHM had very little do do with Sabbath or their style, and plenty to do with the other "Old Wave" bands, but it was notable that the highly talented Diamond Head were cited as "The New Led Zeppelin", and would have drifted into obscurity without Metallica, and it ended up that the mostly talentless Venom were the most influential of all the NW bands after Maiden.
The NWoBHM gave rise to Metallica, who are mostly responsible for the sound and style of a large number of modern metal bands. Metallica cite many, many fairly diverse bands as influences, but certainly, Sabbath's tritonic approach and drop-tuning are notable in their overall style until Load.
You've reminded me that I've mostly confined my explorations to British bands... to be rectified
