i've gotta agree with ondrej on this one, we're talking about equipment that ranges from 20-75 years old here and the life history of each piece of equipment is very very important. i don't have a j-9, but isn't a primary bit of trouble with one usually that someone took it apart and didn't put it back together correctly? this has much more to do with whether a j9 is usable today or not than what it was like when it left the factory.
what i love about FSU equipment i don't think can be described by asking what the best stuff is. it's more... how does this piece of equipment change how i take photographs? i've had very different experiences with my Fed-2 that had light leaks until Jay helped me, my perfect Fed-5c, and my battered Kiev 4am/ J12 setup which has taken some of my best rangefinder pictures to date. i love having multiple FSU cameras(and my argus c3) with me at a variety of focal lengths and seeing how the world changes with each.
if that's how you look at it, it doesn't really matter what is the best j12 out there. my '71 J12 is fine. so is my Fed2 with t-shirt paint on the shutter. i've loved using them.
freestyle photo put it right in their description of photo-taking with a holga. they call it a collaboration between a unique (ie in that every holga has some strange quirk) camera and the photographer. i'm not trying to say a high quality camera like the Fed2 is similar to a Holga, but i'd approach using the two in a similar way. see what you can get out of each camera.