Leica Elmar 50mm f2.8 Type I
Somewhere in the past I told myself that not only did I need a really compact lens so I could put my M3 into my coat pocket when not in use, but that I also needed a 50mm lens. Thus, I purchased a 50mm f/2.8 Leica Elmar.
Now I just wear the camera over my shoulder on a strap like an adult, and prefer 35mm or wider lenses, so the Elmar doesn't get too much use.
I've always found the image quality from the Elmar to be really nice. Though as a major negative is the ease of adjusting f-stops. It is really hard adjust them without screwing up the focus. Not a major issue if you have everything metered out before hand and stick to one stop, but if you have it on an M6, or another body with an internal meter, it can be sort of annoying to deal with.