Contax camera's are very capable and refined machines. The Zeiss lenses are mostly fine pieces of glass. They tend to be heavier and bigger than their Minolta or Olympus counterparts. I own the RTS III, the RTS II, 159MM, 139Q and 2 Aria bodies, and I have owned RX and ST, along with a set of primes. If you want to hear some specifics a particular body, just drop me a line.
On the bodies you mentioned.
I owned the ST and RX, but sold them when I got the RTS III. I bought the RTS III because it has a 100% viewfinder. But it is heavy, unfit for travel. For that I have the pair of Aria's. Small and light little machines with all the features I need, albeit with a 95% viewfinder. Out of the ST and RX, I liked the RX the best. It has the most well damped shutter. I would have kept it, if I had to chose only 1 body.
I have handled the 167MT, but I do not like the lay-out and the weird buttons it uses. In that respect it is a bit of an outlyer in the Contax range.
If you consider the RTS, I would strongly recommend the RTS II instead. It has the same exterior but has some improvements, among others viewfinder coverage of 97% instead of 92%.
The S2 and S2b are "snob" camera's in my view, overpriced for what they offer, unless electronical assistance in photography is a pest to you.
Depending on your use (travel etc) I would consider, the Aria if you want automated film advance, and the 159MM if you can do without. They both share an AEL button, rubberized grip, the grippy bumb on the right hand side, and a 1/4000 top shutter speed, all features I find necessary. I you want something bit more hefty I would recommend the RX(II). It has a silky smooth shutter.
http://cdegroot.com/photo-contax/
Good luck.