One of the very best 21mm lenses ever made is the Zeiss Biogon 21mm f2.8 designed for the Contax G Series of cameras. In a world where superlatives are vastly overused and thus devalued, this lens is genuinely stunning.
This lens is sharp corner to corner including wide open at f2.8. It exhibits non of the softness of cheaper lenses used wide open nor indeed some of its much more expensive rivals for that matter.
The electronic focusing system in my Contax G2 on which I use this lens is a boon as this 21mm is so sharp you still have to be able to focus it accurately at open aperture and cannot rely on un-sharpness to hide focusing inaccuracies as in most other 21mm lenses.
All of the pictures here were made with the lens wide open at f2.8 just to illustrate its resolving power.
Zeiss lenses are renowned for they colour rendering which imbue images with a 3D pop and this 21mm is no exception.
They are also known for their ability to capture micro contrast as you can see here in this scene which will be instantly recognisable to Harry Potter fans.
The 21mm also exhibits no visible distortion and bear in mind there is no possible in camera firmware correction going on here because there is no in-camera firmware as the camera is analogue not digital.
All in all this is a beautiful bit of glass that combines rich gradation, clear, three-dimensional reproduction along with high colour saturation. It's a real goodie.