I have elmar 5cm f3.5 in M mount and summicron 50 type III. Both are great and sharp lenses, good with flare as well (but summicron handles flare better). Elmar is smaller, but comparing to elmar in screw mount it is big. Good thing about M mount elmar is that it takes normal 39 filters, and it is aperture clicks. You cannot go wrong with any of those if they are in good condition. Summar 50 is great (I have it) - but not as only lens for Leica (specific character). I had before 50mm f3.5 industar collapsible - also nice and sharp, and super cheap.
Any 50mm lens for Leica is good - question is only how much $$$ you are ready to spend .