Victor, I suggest to go for price as all of cameras you listed are good. I'd choose from 35RC (if small size is main requirement, and note limited speeds) or KAS2. Electro is very nice though needs battery(-ies) and you can't deal with speed directly.
Can't say anything about Canonet, except I skipped it because of hype and according prices. If you watch ebay carefully, you should be able to get one below extraorbital price tag they tend to fetch, at least some year ago, haven't watched them since then as any other sub-f2 FL RF does same as Canonet, in fact. If your heart cries for Canonet, take a look at non-GIII Canonet model. Most of bidders madly go for GIII lettering while they are nearly identical cameras.
Zorki 4K, as any of FSU cameras....I'd go for them only in case I'd use it with 28mm lens. In case of 40-50mm I'd stay away from FSU cameras.
Just don't pay too much for any of them. Most likely you will have to do some work to get them fully working - I have, so I look for listings not attracting power bidders.
KAS2 with frozen shutter could be got for initial about-$10 + shipping, just make sure it's not dropped or otherwise damaged.
Probably take a look at Yashica Lynx 5000/5000e - same story as with KAS2, just different looks from Yashinon vs Hexanon and much better shutter release.