The brightness of a rangefinder camera's viewfinder is not the only thing that matters.
Leica's might not be the best in all (or any) of the properties that make a good rangefinder (accuracy, brightness, patch contrast, flare, how easy does it go out of alignment, how important the eye position is...), but they have an all around nicely designed rangefinder system.
Some people happen to like M3's magnification and think that because they like its magnification then the M3's rangefinder must be "the best". Good luck telling them they are "wrong"...