The best Leica 50mm right now is the new Summilux which apparently performs slightly better than the Summicron at all apertures ;-).
Originally Posted by haris
If you are going to get a 50/f2, get a good example of the slightly older black Summicron version without a built-in hood, but WITH the focusing lever. My understanding is that the optics in this version are the same as the current model with the built-in hood, but in a lighter more compact lens form, and this lens is generally cheaper on the used market than the latest model. The even earlier black 50mm without hood or focusing lever is apparently a slightly older optical design.