All revealing entries here.
My only hesitation is the Yashica body: the 'X' (electronic) is more reliable than the simple 'Electro' (mechanical). I have had problems with that. But the Yashica lenses are tops.
On the other hand, there definitely was reason for taking a full four years of R&D in order to bring the Spotmatic to the marketplace. It is truly a refined instrument. But I am happiest to see that its predecessor, the H1a and S series, get its due. For me, at least, the aesthetic appeal of that earlier body is first in line as the most beautiful ever created. And, to boot, its mechanics were excellent and durable and desirable to behold.
Not as 'sexy', the Mamiya DTL series was, nevertheless, reliable and its Mamiya lenses also excellent. The Fuji ST series was one of the unheralded champions of the M42 genre, also.
These cameras mentioned by posters were mostly all great, and, yes, AgX, even if you have 'only' a Zenit, if it got quality control and works well, you also have an excellent, albeit limited in function, camera. The Helios 2/58 is a truly great lens.
But, not mentioned are the Ricoh TLS and various Chinon bodies. All have durability and functions that were noteworthy and robust and just plain fun to work with. - David Lyga