Leica thread mount lens buyer's guide:
http://www.cameraquest.com/ltmlens.htm

Stay away from Russian lenses.

My recommendation would be a coated post-war Elmar.
This lens is much easier to find in clean condition than a Summitar, Summarit or collapsible Summicron, which are often plagued by haze, scratches and coating damage. Stopped down a bit, the Elmar is a very decent performer.
Bigges drawback are the ergonomics (aperture ring).

Modern collapsible alternatives are the CV Heliar 3,5/50, which has excellent optical qualities, but is difficult to find, as it was a kind of limited edition item. Another very compact (but rigid and relatively heavy) alternative is the CV Skopar 2,5/50, which is infamous for its sample variation (I can testify from personal experience). A good copy is a decent performer, but nothing spectacular.

There are more rigid alternatives, but they are big/heavy, like the Nokton 1,5/50, or almost impossible to find (= expensive) because of their scarcity like 1999 special edition version Summicron/Summilux in M39 or the first rigid Summicron in M39.