It is quite hard to choose a Leica M lens, partly because of the variety of lenses, and the variety in prices. The Summicron is the best all around choice, and is the one which I prefer to use on my M cameras. The Summitar is a great lens, but it is prone to separation in the front elements, has rather swirly bokeh, and is quite prone to flaring, even on overcast days. The less expensive Summar shares the flaring problem, but usually is good at delivering that Leica "glow" you often hear of. The Summicron seems to have the best balance of sharpness and contrast, and has the most modern design.
When looking for a Summicron, be on the lookout for damage to the coating on the front element, and haze inside the rear element. Cleaning marks and scratches do not seem to affect the images much, unless they are severe. Haze on the inside of the lens increases "glow", and decreases contrast. The haze is almost impossible to remove without taking the internal lens coating with it.
A good alternative to the Summicron is the 50/2 Heliar made by Voigtlander. This is a very nice lens which also has a classic look, and it will match an M3 nicely. I use a black Heliar on my black paint M3, the camera and lens look made for each other, and the photos are quite nice. The quality of the materials and workmanship on the Heliar lens are much better than what I have found on other Voigtlander lenses.