I have Fed 1,2,3,5, Zorki 3M
Judging from the models I have:
fed 1: the smallest and build quality is superb! drawback: bottom film load. First time I used it film was loaded with ease, but second time it got stuck and was not advancing. Might happen with any film camera I guess. One of the disadvantages is that earlier models had different distance from film to lens which requires to adjust some newer lenses. In later fed 1 it has been changed as far as I know
fed 2: great build quality, widest rangefinder base. The one I was shooting the most since I got it
fed 3 (I have only 1 of those): poor build quality and it arrived with some problems (missing screws, not functioning bulb mode). Look carefully at the pictures when you buy any camera on ebay.
fed 5B (the one without exposure meter): amazing build quality, looks similar to fed 3. It is also bigger than fed 1 and 2 (much higher). My only complaint with it is a bit too tight cover (need to make an effort to take it off).
Zorki 3M: very big and bright viewfinder! I like it a lot. That is the camera I have with me right now. Might even become my primary cameras. Often later Zorkis come with more advanced lenses such as Jupiters (mine came with Jupiter-8).
I would not part with any of those cameras and would use all of them based on particular situation. For instance if I need smth small I would take my fed 1 with a collapsible lens it came with. If I'm going to shoot in dark conditions I would rather use my fed 2 or Zorki 3M with a brighter lens. Actually my fed5B and fed1 viewfinder is pretty good as well.
To start I would probably buy a fed 2 since it is cheap. Zorki 3M and fed 1 are rather exensive (comparing to fed 2). Fed 5 is cheap as well.
The lenses I'm using the most are: Jupiter-3 50/1.5, Jupiter-8 50/2, Industar 61L/D 50/2.8 I shoot wide open most of the time and had no issues with either of those.
fed 2 often come with Industar 26m 50/2.8 which performs fine as well (at least to me), the collapsible Industar 10 or Industar 22 50/3.5 produced great pictures as well.
Hope that helps
I've heard Kievs are great and Leningrad is a great camera, but did not try either of those.