I don't think that Leica and Zeiss lenses generally are better than other lenses. It simply so that some lenses are better than others.
It may be that a series of lenses have desirable features, like having good manual focus and smooth mechanical operation. Some other lenses may have more advanced antireflex coating.
MTF curves give very good information about sharpness in the focal plane, but little about out of focus images.
If we look at Zeiss lenses for instance the 21/2.8 seems to be really excellent from sharpness point but the 18/3.5 is far less impressive, according to Diglloyd. The Zeiss 25/2.8 was really not very good from sharpness point of view but may nevertheless be attractive. I would suggest to try out the lenses you are interested in as rentals.
A general characteristic of Zeiss lenses seems to be that they are not optimized for sharpness in the corners.
A big part of the appeal of the Coastal 60/4 images comes from a complete absence of any color errors of any kind, which leads to an unusual clarity. No other lens can match its color correction
not that it is better than Leica or Zeiss lenses
on a D800E I shall use Zeiss and Leica R lenses