50-ties have been my main lens for quite a while now. I seldomly use my 35 and as for my 28 and 90 ... never.
I have the latest cron, which is good at everything, but perfect in none.... good for family type shots and I guess slides.
The 1955 50mm summicron collapsible is possibly the best lens I have for I love to shoot with Tri-x or Agfa apx 100 this lens gives the perfect noir look.
For sharpness nothing beats a dual focus....
For slides I guess the latest version are always thze best.
A real cuty, and great result overall be it B&W, slide or colour negative is the latest 2.8 Elmar.
I have the luxury of owning 4 M cameras, the MP, M6ttl and 2 M4's so I can distribute hte film with the appropriate lenses, all according to personal taste. Most used is the MP with pre-current 50 cron or the Lux, and the M4 with the collapsible cron, which by the way is in perfect condition.
I allways have the M6 with the elmar with me, but coming down to the amount of shots it is not as heavily used as the afore mentioned combinations.
I find the 50 to provide the most natural result, step back or take two steps forward to simulate the 35 or 75, 28 or wider is to wide, distorts even if only minimally and I've never been a fan of telelenses.. As mentioned before you must adapt to the 50, once having done so you will rarely need anything else...
I just bought the 50 Zeiss, just to try out and it is very much like a late cron, I at least cannot see any difference between them, but for the price of a second hand cron I would alwaxs stick to leica, it just feels good... and having a leica camera or cameras you will always want one in the end.