The 50s i've owned, in order:
Summilux, pre-ASPH / original Zeiss Sonnar 1.5 / Jupiter-3 / DR Summicron / Zeiss ZM Planar / Summilux-ASPH / current Summicron.
[Jeez. That's ridiculous. I don't even really like rangefinders....]
Anyway, if i had no lens, and had to buy one now, i'd think about these:
Zeiss C-Sonnar, Nokton, ZM Planar.
The Summilux-ASPH was certainly the best of them, but i wouldn't spend that kind of money on another one. I'd love to have a C-Sonnar, but i do worry about the focus shift. As well, i'd want my only 50 to be sharp, and from what i've read, the Sonnar isn't really 'about that.' The Nokton is something i've always been interested in, because the pictures are so nice, but other lenses somehow were more alluring. Probably because of pedigree. I was happy with the Planar, but wanted more speed. So far, i have not been impressed with my current Summicron, although i haven't used it often enough, and in good light. I was not impressed with the DR Summicron. The Planar was sharper. The pre-ASPH Summilux was very nice. I'm not sure why i don't think of that experience more fondly. Maybe because it was so long ago and i can't remember specific pictures i shot with it. But, it was my first rangefinder lens, and so that may have just been a matter of me getting to grips with RFs (still haven't). It's not a fault of the lens. The imagery is nice - classic, with nice bokeh, and it's sharp enough.....
My only advice is that if you're talking about a first/only 50mm lens, get one that's at least an F2. There's never enough light.