My suggestion is a modern f2 50mm lens. The ZM Planar is a good one, as is the Konica 50mm f2. From Leica, the 4th and 5th versions of the 50mm Summicron (the same optically, but the 4th has a focus tab, the 5th has a wide focusing ring).
I don't recommend the older Summicrons because very few that I have seen are in good enough condition to justify the insane prices they command. Most Collapsible, Dual Range and Rigid Summicrons have scratched up front elements and often have haze in the insides.
The Lenses I recommend, the modern 50 Summicrons and the ZM and Konica 50/2 lenses, are optically very similar, all very sharp, all well built, and all new enough that a used one will be in good condition. The ZM is cheap enough to buy new!
I have a ZM Sonnar. It is a cool lens, but not a good general purpose lens. Its not very sharp across the whole picture unless stopped down, but is very sharp in center. I've had pics of buildings I shot at f5.6 ruined by that. The Tabbed Summicron I also have is sharp everywhere, even wide open. The Sonnar also suffers focus shift issues. Mine was an older one made to focus at f2.8 and below. It is not sharp at f2 or 1.5 because the plane of focus is so far off what your rangefinder shows at those apertures. Newer ones are calibrated at f1.5 and are sharp at that setting, but unusable until about f5.6 because you cannot focus accurately at f2, 2,8, and 4 with one calibrated for f1.5. I know you'll now get a bunch of suggestions for techniques for 'making it work' at those apertures. I'd suggest that guessing focus, which is essentially what will be suggested, is not a good way to work, especially if you're new to shooting rangefinders. You need gear that "Just Works". Anything else will frustrate you.
