It’s good to be king, unless nobody knows it.
Right at this moment, the grandest 21mm lens design ever made is sleeping in a drawer somewhere at Zeiss. This is really a shame, because it deserves its chance to shine in the market and let it be known as the 21mm king. One of my fondest hopes is that Zeiss is working hard to port this design to the F bayonet in order to release it in the ZF line shortly.
The reference specifications for the Contax MM 2.8/21 Distagon T* are still available here.
What’s really astounding from this design is the raw performance it’s capable of. Typically the resolution of wide-angle lenses plummets at the 135-format corners, right down a cliff, where the lighting angle is too much for spherical lenses to handle. Second only to the MM Distagon-21 are the two Leica designs: the 19mm Elmarit-R ASPH still could not resolve more than 40lppm@20% in the far corner at any aperture, and the 21mm Elmarit-M ASPH that achieves high resolution at the corner only for sagittal lines (i.e.: astigmatic).