I would say that, among the Leica lenses, the Elmar 50/3.5 is the classic. The following summars, summarits and even the summicron were faster and better in some ways, but it is very difficult to find a good copy due to their soft coatings. Also, although you can say the collapsible Summicron is a fine lens, it is not nearly as good as the DR summicron, and also not nearly as compact and collapsible as the Elmar 50/3.5. So, it is more of an intermediate step, which culminated in the rigid body M-mount summicrons, rather than a fully realized vision.
In favor of the Elmar 50/3.5 -- it is fine at all apertures. It is very, very compact, and the combo of Leica III and 50/3.5 is beyond classic. It has its own, special "graphite pencil" signature. It is very sharp. And, it is a 1930s design, with funny quirks (front aperture slider) that give it a special flavor. The Elmar 35/3.5 is also classic, but needs an additional viewfinder, which is kludgey.
Among LTMs, I would also single out the Canon 50/1.8 and 50/1.5. I think I would go for a Canon sonnar 1.5 rather than any f/2 or faster Leica in LTM. Just my opinion.