(EDIT) Think I should leave brand (Leica M) out of the issue. Because the theory applies to any rangefinder (except for those old fashioned ones with bellows).
Recently heard a friend make this claim, as though he was expounding a Nobel prize winning theory; and that he is the one who knows everything.
Rangefinder lens give best image quality as the rear element of the lens is nearer to sensor/film plane. Because in the rangefinder design, there is no SLR mirror that would have needed more space in between
What is your view or experience?
Being an owner of more than 1 rangefinder, I doubt this claim.
I have not discovered such image superiority from my photos taken with the rangefinders.
I have not discovered such image superiority from my photos taken with all my other rangefinder cameras of various brands.
Conversely, I have not discovered that photos taken with film SLRs or digital SLRs are of inferior image quality because of the extra space needed to accommodate the SLR mirror. (EDIT)