How about a collapsible Leica Summicron in LTM?
Nicely sharp and back in its day, became a standard for performance. I also understand it was a lens often used by Henri Cartier-Bresson.
Though take care with it if you go with the old Summicron, even though my example is clear and pretty clean, it seems to catch more flare than my other lenses. (Of course, I could use a lens hood more than I do.)