I am sure, that this post, as easy as it is, will bring confusing and inclnclusive results. I have voted M7, but I wish I could vote Zeiss Ikon, which in fact, and quite an obvious fact, is overall the best M mount camera for what it does.
However, this answer is inequivocably correct, if the question is: what is the best M mount camera for typical rangefinder application shooting - e.g. photo journalism, street and travel photography.
The answer will be different, if the question is: which is the most sexy M mount camera for fondling, or : which M mount camera will be best for tripod shooting in the anctartic, or which one is best for driving nails, or which one fits best in my pocket...