I've shot all the FSU 50s, quite a bit. Here's my take on it. The J-3 is the most classic of them, with the J-8 being almost as good. These Sonnar types have warm color rendition, and are sharp. Their out of focus areas look great.
Second, I like the Industar 61. I see no reason to get the L/D over the regular version. This lens, my Russian friend tells me, was not considered very good. However, in a lot of testing, I've determined it really is. It is very sharp, has click stops, and great contrast.
I've only had one or two I-26s. I did not like them at all. They were flarey, low contrast, and cheap builds (like all FSU lenses). I sold them.
The Fed-10 or IL-22 are often said to be "just as good as an Elmar" but I don't agree. My copies have had noticeably weaker contrast and are less sharp. Yes, it's nice to get a collapsible lens for $19 or whatever they go for. But then you have a $19 lens. Save for an Elmar and you'll notice a huge difference.