Some basic background on these cameras: The Soviets cloned the Leica II several times, but the two lines that "took" are the FED and the Zorki. The first iterations of these are virtually identical, and AFAIK there's not much reason to prefer a 1st-generation FED vs. a 1st-generation Zorki. Each line then evolved more-or-less independently.
If you want something that's very old-fashioned, then a FED 1, FED 2, Zorki 1, or Zorki 2 will probably fit the bill, and all will be pretty similar, with the big caveat that cameras of this age will all have their quirks (on top of the substantial sample-to-sample variability out of the factory). The Commie Cameras site referenced above will give you some basic information on the model-to-model differences. FWIW, I own a FED 2 with a collapsible lens that I use as a take-anywhere camera, since it can fit in a jacket pocket (but not in jeans pockets). It works reasonably well in this respect, and I've taken a few decent photos with it. Its rangefinder image is displaced vertically, which makes focusing a bit weird, since the images line up correctly horizontally but not vertically, but otherwise it works just fine.
If you want something more sophisticated (with a built-in meter, for instance), a more recent model may be in order. I've got a FED 5 that's a capable camera, and the Industar 61L/D lens is pretty sharp. (My sample is very stiff, though; it needs a CLA, but I've never gotten around to doing one.) The FED 5 has a built-in selenium light meter, which works like many hand-held selenium meters, but doesn't require an extra pocket to hold it. It's bulkier than my FED 2, so it's not really a pocketable camera like the FED 2.
Note that some of the older models, particularly in the Zorki line, are bottom-loaders. This design can be a bit awkward, particularly if you want to load film in the field, since it requires extra-long leaders. Cutting a new leader isn't that hard, but you'll either need to do so at home and take pre-cut film with you or take scissors or a knife with you to do it just before loading the camera. Both my FEDs are back-loaders, like most 35mm cameras (although the backs come off entirely). I think the FED 1 was a bottom-loader. I'm not sure when Zorkis changed from bottom- to back-loading.