Do you need a certain speed, or are you after a particular 'look'?
The 'cron is close to optical perfection in a 50 - flat field, no focus shift, outstanding wide open, great sharpness across the field. The Zeiss Planar performs virtually the same, with a bit more contrast and flare resistance. If F2 is all you need, look no further.
The C-Sonnar is the most compact and has the most 'character' of the three. Lots of focus shift and field curvature at wide apertures. Some people love it for portraits... others can't stand its quirks. Stopped down to F8, it performs like any other modern 50. As a Sonnar with Zeiss coatings, it has the best flare resistance of the group. Most people like the bokeh from this lens.
The Nokton 1.1 has good sharpness wide open, though not very strong in the corners and noticeable vignetting. On film you'll be able to see focus shift at F2.8 to F4, so I usually avoid those. Moderate contrast, and good flare resistance. Lots of fun if you don't mind the bulk/weight and love shallow DOF or low-light shooting. Bokeh is a bit unpredictable; you'll see bright edges on the circles sometimes.
A good compromise is the Nokton 1.5, costs around $500, no focus shift, and good copies can rival a Summilux ASPH in performance. The M-Hexanon and Zeiss Planar are good alternatives to the Summicron (very close performance yet much cheaper).