Zeiss ZM 15mm f/2.8 Distagon on Leica M9
The ZM 15/2.8 Distagon utilizes exotic types of glass including barium dense flint with a high refractive index and fluor crown with anomalous partial dispersion, as well as an aspheric lens element (unusual for Zeiss designs). Zeiss went all out with this design, hence it is the most expensive lens in the ZM lineup (about $4600), and don’t forget the optical viewfinder (about $435). You definitely do not want to scratch the (supplied) center filter, because it’s over $500 alone!