I've got quite a few 50s, and have tried more, so perhaps my experiences will be of some value. Bottom line is that you can hardly go wrong with most of the modern RF 50s. Even the older ones are quite fine stopped down to f/4 or 5.6 or narrower. That said, here are a few comments.
The 1980s and later 50/2 Summicrons (current, or the penultimate one with the tab, which has the same optics) are about as good as it gets. The current formula is pretty sharp even wide open:
http://users.2alpha.com/~pklein/currentpics/mischa_emphatic.htm
Stopped down a little, if you can hold the camera steady enough, you'll have all the detail you could ever get in 35mm. Web pics don't really show sharpness, but in these next two, you can see the individual granite grains on the original Provia slides:
http://users.2alpha.com/~pklein/california/4-15HMooreBW.htm
http://users.2alpha.com/~pklein/california/4-24JTreeHandsUp.htm
The newer lenses are a little harsher and have a *slightly* more wiry bokeh than the older Summicrons. I think the trade-off is well worth it. The only other flaw the current formula 'cron is that it's prone to refllect a pinpoint highlight or light source somewhere else in the picture.
The older 'Crons are staggeringly good, too--just a little mellower than the current formula. I have a 50 DR Summicron (the "rigid" is the same formula). It has slightly less crisp edges than the latest formula at wider openings and away from the center. It's a little lower contrast. But the DR has an "it" quality that defines the "Leica look" to me. I can't quite define it, but I know it when I see it:
http://users.2alpha.com/~pklein/temp/25JoyceCraig.jpg (wide open)
http://gallery.leica-users.org/album225/37ThinkerBW (wide open)
http://gallery.leica-users.org/album225/01OldShedTree
The 50 Nokton is a decidedly "today" lens. It is astonishingly sharp even wide open, the trade-off being a harsh and wiry bokeh that can also emphasize the edges of out of focus highlights. In terms of giving a sharp, clear image when the lights are low, it beats out the old 50 Summilux all but dead-center, and even there, it's close. I bought one used for $260, and I have no regrets. I find the overall look and bokeh quite similar to the Leica 35/1.4 Summilux ASPH, and they make a good "matched pair" for shooting available light. Yes, the Leica is a bit better, but the difference is just noticeable with 400 ISO film at 8x10. Stopped down, the 50 Summicron is staggeringly good, whereas the Nokton is just very, very good. The Nokton is probably the best value for money in a high-speed 50mm lens. Some Nokton shots:
http://gallery.leica-users.org/Rachel-Scott/1_6231_01 (wide open)
http://gallery.leica-users.org/Rachel-Scott/3_6233_18 (probably f/8 or 11)
http://gallery.leica-users.org/Rachel-Scott/1_6231_36 (probably f/4 or 5.6)
http://users.2alpha.com/~pklein/lhsa2002/30ted.htm (wide open, 1/8 second)
When I travel, I take a 50 'cron with me if I'm mostly going to be outdoors. If I know I'm going to need f/1.5, I take the Nokton, and happily shoot it everywhere.
The 50 Planar: I've only handled one briefly and shot a couple of pictures. I was quite happy with the best shot. It was very pleasing, and the lens handled contrasty light very well:
http://gallery.leica-users.org/Vancouver-LUG-2005/15TomMarilyn
The Jupiter-8: I have one that came with my 1962 Zorki-4. Mine is decent, but I don't know if it is an especially good one or not. At f/2 - f/4, it has a "retro" 1930s look that is very pleasing for people. It doesn't quite resolve individual hairs in a head-and shoulders portrait, but it doesn't do so in a beautiful way!
http://users.2alpha.com/~pklein/ (me, f/4, 1/30)
http://users.2alpha.com/~pklein/temp/16RabbitBoy.jpg (f/4.5, 1/60)
Stopped down more, it is a decent lens, but the Summicron beats it for both resolution and contrast. The J-8 needs a good hood, as it can exhibit "wash-out" flare easily. Also, its Sonnar design has a tendency to show OOF highlights as discs with a bright edge and a darker center, and can double up OOF lines (the notorious "ni-sen" bokeh). All Sonnar clones and derivatives do this to some extent. I used to have a 50/1.4 Nikkor, and I've seen the same thing in 50/1.4 Canon shots.
I'd say that the latter Japanese Sonnar deriviatives are objectively better lenses than my Jupiter-8, but the J-8 is a really good "people" lens. The gentle smoothing of skin texture and detail in facial lines is often very nice. The problem with many Soviet lenses is that they just don't focus right on a Leica. Either they are correct close-up or at infinity, but not both. The two J-8 shots above were taken on a Zorki, not a Leica--my J-8 is "off" on my Leica M bodies. That's my experience, your mileage may vary.
I have an Industar-61 LD which doesn't focus correctly on *either* my Leica or my Zorki. But I'm sufficiently impressed with things I've (accidentally) gotten in focus that I'm probably going to pick up another one or two until I find one I can use. It is a very sharp, contrasty lens.
If I could have only one lens, it would probably be a tabbed 50/2 Summicron. Oh yeah, if money was no object, I would have a 50/1.4 Leica Summilux ASPH, but money is an object, so. . .
Hope all this is useful. Note that I've posted f-stops etc. when I remember what they were. If I didn't say, I don't remember.
--Peter
