Hallo! In a way, this is one of those unanswerable questions, but I'd suggest that the obvious choice for a first screw-mount FSU camera would be either the Zorki 4K or FED 2. Both are widely available and cheap. A slightly more esoteric choice might be a Zorki 6 or the Leica-like Zorki/FED 1.
All these cameras have avid partisans. Personally I favour FEDs; the FED 2 is a classic design, robustly built with beautiful handling and a fine, long-base RF. The limited range of shutterspeeds (1/30th -1/500) are seldom a problem in general use, but the viewfinders are small and rather dull, although individual cameras can vary quite a lot.
Many people love the Zorki-4. A large, bright viewfinder and a wider range of shutter speeds (1 -1000th) are strong plus points, but I never took to the camera, partly on aesthetic grounds. The Zorki 6 offers a similar specification to the FED 2, with a much better finder and a conventional door to load film. But the latter can sometimes prove a weak point, prone to lightleaks and accidental openings. It's not hard to remedy these quirks - and many users have no trouble at all - but such problems sapped my confidence in an otherwise delightful camera.
If you want something really interesting, go for the Zorki or FED 1 - basically a Barnack Leica. Unique, peerless and perfect in its way.
There are many other possibilities - and you'll probably end up owning the lot!
Incidentally, a good e-bay seller - like the excellent Alex-photo - is as trustworthy as anyone.
All the best, Ian