Over the course of time, I've spent a lot of money and time on different 50mm lenses. So, here's the distilled essence of what I've learned.
First: Condition is key. You need to find as clean a lens as you can, especially for some of these, like the Leica Summicron collapsible.
Second: Some lenses are capable of focusing very closely (Nikkors), but lose rangefinder coupling after 1m. You can crudely file the mount, but a more elegant solution is to send it to someone like Don Goldberg (DAG) who will mill the mount to allow RF coupling to 0.7m (sometimes the RF cam will go all the way down to 0.6m).
Nikkor 50mm f/2 HC - I had a very early Tokyo version. Sent it to DAG who overhauled it and milled the mount to 0.7m. Wide open, it was very soft with very low contrast and a lot of vignetting. Stopped down, it sharpens up fine, but this wide open performance did not thrill me, so it was sold. This lens is extremely well built, and it is small but dense.
Nikkor 50mm f/1.4 SC - I'm trying again with a "Japan" version in chrome with a black aperture scale. This is a later version of the lens. My lens is incredibly clean for a 60 year old lens. I've shot film with it, but the film has yet to be developed. Anyway, I've already sent it to DAG to get it relubed (it's a bit stiff), and to have the mount milled. Built equally well as the f/2 version.
Canon 50mm 1.4 LTM. A lot of people love this lens. I'm rather neutral about it. The lens is good, but unexciting. It has no real flaws besides being bulky and heavy. The original Canon rear caps are a hazard. They are too shallow. If you turn the focus ring away from infinity with the cap mounted you WILL ruin the RF cam's accuracy. I had mine overhauled by Clarence Gass, and now it is spot on wide open at 1m. This lens reminds me of the Nikkor 50mm 1.8 AIS on my Nikon SLRs. A very good lens, but somehow unexciting.
Leica Summicron collapsible. You've read that the front element is as soft as chalk and easily scratched. It took me a long time, but I finally found one with perfect glass. It did have haze, so I sent it to DAG for his magic. This is a great, and I do mean great pictorial lens. People claim that it is low in contrast. I disagree. It has as much contrast at f/2 as my Canon 1.4 has at f/2. You do need a lens hood to prevent flare, and a good UV filter to protect that super soft front element. Wide open I find moderate contrast, some vignetting, and very good sharpness. It has just the right balance for flattering portraits. Stopped down, it is very sharp. No distortion.
I use the ADVOO close up system on my IIIG with this Summicron, and it is a very decent close up lens too. For the IIIF and earlier, use the SOOKY. For M cameras, use the SOOKY-M to use this lens for closeups.
Summicron 50 f/2 last pre-ASPH version - with pull out hood. This thing is a laser from wide open. Super sharp. Just adjust the aperture to get the depth of field that you want. Leica made a few in LTM in 1999 for the Japanese market. There is one in the RFF ads now for only $1600. No distortion. Unfortunately, Leica gave this a limit of 1m for close focusing. I'm sure that you could have the mount milled, but I sold it and now I just use a regular Summicron M (same version) on my M7. This is a much less expensive solution!
Cosina Voigtlander Heliar 50mm f/2 - similar in rendition to the Summicron collapsible with greater contrast. Nice compact lens. Unfortunately, the front of the lens rotates as you focus. Voigtlander did not repeat the aperture scale on the bottom , so it can be annoying if you want to change the aperture at certain focus distances, when the aperture scale is on the bottom.
Cosina Voigtlander Heliar 50mm f/3.5 - small, skinny lens. It always felt a bit wobbly to me. The aperture ring is way at the front, and it is more easily turned by just turning the lens hood. I know it was reviewed by one of the photo magazines as one of the sharpest 50mm lenses ever. However, with a minimum aperture of f/3.5 there isn't much for a bokeh lover to look at. I sold this one in favor of the non-collapsing f/2 version, mentioned above.
Zeiss C-Sonnar 50mm f/1.5 M - I love this lens! When I sold almost the entirety of my rangefinder system, this was the lens that I kept. I even sold off my Summicron M (I did keep that perfect collapsible Summicron too). OK, there is some focus shift, but you learn to adjust by leaning in and out by 1-2 cm. Mine is optimized for f/2. Good contrast wide open, very high contrast stopped down. I can't see any distortion. The rendition is beautiful with this lens.
Lomography Zenit Jupiter 3+. I just got this, so no developed film yet. As small (maybe even smaller) than the Nikkor 50mm f/2. Amazing that you can get an f/1.5 lens so tiny. I expect some funky bokeh out of this, based on the pictures that Lomography has on their site.
I'm sorry that I can't pick the "best" one for you. I can't even pick the best one for me! Some of these still need to have their film developed before I can judge.