Hi,
with no intension to call the MTF comparison debate nonsense, I want to state a experience of mine with Zeiss lenses from 25 to 300mm and many other brands (Leica, Nikon, Canon, Olympus, Schneider, Minolta).
One big advatage of the Zeiss C/Y lensline is:
All lenses/focal length deliver very high, top class quality. There is no need, to look for little differences in contrast, resolution,...
Choose your Zeiss lens by focal length, speed, weight, volumne, Price. That´s it. You will be satisfied. When the pictures aren´t thrilling, it´s probably on you (I speak of myself ;-)).
There are very few exceptions from this general rule: the old zoom 40-80 has a lot of distorsion, a 2,8/45 Tessar can´t be better than a Planar, but is of small size and weight. And the super wide 21 is especially good, as the Apo 200 and 300. These information you can see in the Zeiss charts and will guess it, when reading the price list.
Nikon (AI, AIS and AF)and the other also have somne great lenses, but not all are on the same high niveau (except Leica M). Some lenses in the other systems come from other manufactures (the Tokina 35-105 was common in nearly all systems, like the Sigma 28-70. Leica R has designs from Minolta, Zeiss, Schneider, Sigma, Angeniuex).
Zeiss delivers high class Zeiss design, Zeiss quality, Zeiss colors, barrels, service over years.
In my eye this is a main argument for Contax/Zeiss.
regards, Stefan