35mm is my favourite, assuming you are a wide angles person you will love it (it is 21mm or so in 35mm photography). If you shoot the normal lens (some do some don't) then the 80mm is quite a performer even at f/2. The 140mm is a fine potrait lens but is NOT a macro (the contax 645 macro is 120mm which is a manual-focus lens. I have heard some say that one should skip the 120mm macro lens and buy an old Hassy macro lens along with the Hassy-> Contax converter, which will still yield a macro and manual focus lens but apparently a cheaper combination, I do not know, I get macro doing some other workflow).
Note that the 1.4x can only be used with the 140mm, 120mm (I think) the 210mm and the 300mm (check that price

), but not with the 80mm. The front elements of the converter move into the back of the lens; the 80mm's elements reach till the back-end of the lens leaving no space for the converter's elements to fit in.
From other users of the lens, I was told that the 210mm is very heavy and that I was blessed to choose the 140mm and the 1.4x combo instead (it still gives f/3.5 maximum). My previous experience with medium format taught me not to choose the 210mm based on my style of shooting and instead choose the 140mm - I found it more useful).
Personally I have used extensively (maybe 60% of my shooting) the 35mm, and lesser the 140mm (20%) and 80mm (20%). I am still not clear why I bought the 45mm. One of the nice things of the system is that out of the box, the camera prints the data about the f-stop, speed, mode (AV etc), 1.4x converter etc at the top of the frame, which is useful is you are considering what to put in your back.
Very recently, I did some enlargements that came from working with these and I am very very pleased (if you were at the travel exhibition this year at the Singapore History Museum, the Cyprus photos were taken with this system).