Regarding the original post:
A Pentax LX is "tropicalized", small, you can find very good lenses at a decent price, you can use Pentax lenses on Pentax cameras of different generations, possibly including some modern digital ones, and if starting a "system" today that would be a good choice.
Against it: said to be noisy (actuation is noisy). Cost (the LX not the system).
A Contax RTS is not small. The Zeiss lenses, besides being very good quality (at a very bad price) can "match" the chromatic response of Zeiss lenses on medium format (Hasselblad, Rollei). That can be useful for highly specialistic work. If money is no object, Contax would be a fine choice. Yashica lenses are also good, but you have no camera with interchangeable finder if that becomes important one day (for tripod work a waist-level finder can be very gentle on your back, not to mention macro, work with a copystand etc).
A Minolta system with SR mount will allow a great, great quality/cost ratio on lenses and cameras, because the SR (also called MD) mount went out of production when Minolta switched to autofocus and had to change bayonet. You can build a complete system of high quality and low (sometimes very low) cost but autofocus is something you have to give up (not a problem here). A Minolta XM would be the equivalent of the Pentax LX (without TTL flash) but you can also find plenty of mechanical cameras at a ridiculous price.
A Minolta system with the new AF mount will allow you to use the same lenses on the Sony Alpha digital cameras (or Minolta digital cameras) besides on Minolta AF cameras. But the lenses are not so inexpensive as the SR mount ones.
A Nikon system would be an interesting choice because you can use old lenses on new cameras (with some incompatibilities here and there) and generally speaking Nikon cameras and lenses are very robust. They would be the obvious choice if you think you are going to stress and abuse your cameras. Until the Eighties or so, Nikon lenses were not just very robust but very repairable, often with each lens element mounted with its own collar, which made easy to realign a lens after a shock. This had a price (cost, old designs, weight, and size). I don't think this can be said of the autofocus production at all. If you wear glasses a Nikon F3 HP would be a very sensible choice. Everything branded Nikon still maintains a "badge premium" and if you don't like paying for "brand" is probably not your optimal choice.
Leica: see Contax/Zeiss above regarding high quality and high cost, multiply by two, and see Nikon above regarding paying for badge, multiply by 4.
Olympus: the OM-1, OM-2 and OM-4 are very interesting for their miniaturization, those cameras are even smaller than the smallest Pentaxes and especially the lenses were normally designed to be smaller and lighter than the competition. Too small might become uncomfortable for your hands though, especially if they are big, if you use gloves etc. The shutter speed collar around the lens mount is something that you can like or dislike, beware of that.
If I were you I would take the Pentax route (for the highest versatility at a decent price) or the Minolta SR route (for the best quality/cost) ratio.
Canons: like is the case with Minolta, Canon had to change their bayonet when they went Autofocus. The FD mount arrives up to early Eighties. As with Minolta you can find plenty of cameras and lenses at a relatively low price. My general idea is that the average lens quality is not on par with the Minolta, while the average camera quality is and you can add to your system a Canon F-1, F-1n which is certainly more modern than a Minolta XM.
In one line, I would go for an OLD system (Minolta SR or Canon FD) unless you feel you need features like autofocus, matrix metering, and other devil-inspired features because these out-of-production lenses and cameras offer a high quality at a very low cost.