...What lens do you recommend to that "system"? I now that Zeiss are good but probably also expensive.
Everyone has different tastes so maybe there's no point recommending any lenses unless you tell us what types of pictures you like to take, and probably your budget too. I've had quite a few Yashica ML and Contax lenses over the years and found most of the ML lenses quite good but never really as good as the Contax equivalents, maybe some of the 50mm lenses were pretty close.
I had a terrible ML 200mm lens but maybe it was a dud, I only had one copy. That's probably the worst Yashica ML lens I've ever had.
The ML 21mm is very good stopped down, not great wide open, but they are not cheap anymore.
The ML 24 is quite good too, I think they are reasonably priced.
The ML 2.8/55 macro is very good, but again there are better macros from Zeiss, but it's a nice lens and certainly one I would recommend.
The ML 2/50 is good considering it's only worth 10 or 20 dollars, but the way I look at photography is that my time is worth more than that so if I go to the effort to take a particular picture I prefer to think I'm doing it with the best tools for the job that I can afford, and I would suggest that most people can afford more than $20 dollars for a lens. I don't recall if it's rendering/bokeh is particularly jittery which can be an advantage if used the right way. None of the Yashica ML or Contax 50mm lenses strike me as having soft background bokeh.
The ML 1.4/50 has a similar rendering to the Contax/Zeiss 1.4/50 but the ML has more purple fringing and is a little less sharp at every aperture. It's not a bad lens but I find I never use it any more because I prefer the Contax 1.4/50.
I love the Contax 2/135 (I've had two over the years) although this is not a cheap lens. It is very sharp even wide open and has possibly the softest background bokeh of any lens. It really is superb for photographing people, however it is not as well corrected as modern equivalents, nor is it priced that way. I suspect the cine guys would love it too, because of it's bokeh, and I've done some timelapse with it on a DSLR with stepper motor controlled focus which were very effective, again because of the sharpness combined with the soft bokeh.
I've had the Contax 1.4/85 but I didn't love that lens, maybe others do.
Same for the Contax 2.8/180.
The Contax 2.8/28mm lenses are superb and not too expensive. I definitely recommend the Contax 2.8/28 if you like that focal length.