I would be cautious to decide for a certain Contax system just by the criteria "which has the best lens".
I think we all know, that no matter what system will be yours, it will have excellent lenses with it. There might be differences if you would compare different Zeiss lenses to each other, but with Zeiss quality in general, we are talking about very very small differences.
In my opinion you need to approach this decision from a different perspective. To get the right system for you, you should first think what at all you want to take pictures of.
See for a detailed comparison my comment in the Why Contax and which Contax for your needs.
You just will be happy, if you find the system you will really use often. After you decided that, you should think about which lens will be the best for you.
Look also in the comments in News/info folder on the left. There you will see some comments about the common mis-interpretation of MTF data and the criteria to really explore the Zeiss quality.
After reading this, you will understand why the decision for this or that Contax/Zeiss-System should not be mainly driven by MTF data or potentially better lens design.
At the end of the day "small" factors of which a newcomer does not think of in the first hand are much more important for great pictures with axcellent image quality, especially the right light and the combination of the light with the subject. Otherwise your image will look like taken with a 49USD lens on sale.
hope that helps