Strickly in RELIABILITY terms:
No dout: The W123 240D (without power windows, power locks,
A/C and fitted with the regular 4-speed manual)
Market specifications and climate regions might change the final view on this subject, but no doubt that the 240D is virtually indestructible.
I have to disagree about the comment about "any well maintained M-B is realiable". Some models, even if you mortage your house to keep them in top shape, just plainly cannot be trusted (1992-1994 W140s come to mind).
For all 240D lovers, here is a pick from the original 1977 German brochure: