I read the OP's inquiry as intended to emphasize the quintessential qualities of a Leica M as an overall package. The qualities that come to mind are simplicity, all mechanical operation, compact size, refinement, uncluttered and simple viewfinder, quiet shutter, and excellent lenses.
When I think of a classic Leica M, I don't necessarily think of extreme toughness (rangefinders can be knocked out alignment fairly easily), large size, heavy weight, LED diodes, automatic exposure, etc.
Viewed in this light, I think Olympus OM-1. About the same size as a Leica M, viewfinder is completely uncluttered (except for very small match needle display), very quiet and well damped shutter and mirror, all mechanical, well built, nice feel, and excellent lenses.
While some have suggested the Pentax MX (also a very fine camera), the light diodes in the viewfinder and less well damped mirror/shutter assembly make it a bit less Leica like IMHO.