The SMCP-K 50mm f/1.2 - I have had this lens and worked with it for years, I know the optical proclivities of my own copy of it and find the lens to be easily in my top 10 50mm lenses ever made. I use it often as it is an excellent lens for portraiture not just because of its speed, but when it is stopped down it produces the "soft-sharpness" that some of the older generation of sought after large format lenses produce. The SMCP-K 50mm f/1.2 it isn't as bitingly contrasty as some of the later and current FA and DA primes*, but it also doesn't have the same degree of optical aberration. In fact, the Pentax 50mm f/1.2 has measurably less axial CA than the late versions of the Canon EF 50mm f/1.2 - not too shabby for a lens designed in the mid 1970s.
With Rangefinder lenses I have a preference for the Leica Noctilux 50mm f/1.0 - the omission of an aspherical element in the optical design leads to uncorrected spherical aberration which leads to its unpredictable love it or hate it optical qualities. The Leica Noctilux 50mm f/0.95 is sharper but it does not have the signature of 50mm f/1.0 Noctilux. When I don't have enough space in my bag for the noctilux, I pick the Leica pre-asph summicron 50mm f/2 [ which has similar optical properties to the sans ASPH Noctilux] which gets paired with a Leica APO-Summicron 90mm f/2.
Pentax K5IIs - SMCP-K 50mm f/1.2 ISO 80 1/6400th@f/1.2
*which is one of the primary reasons why I don't think Pentax will ever produce an AF 50mm f/1.2 lens due to the inherently low contrast such lenses tend to have. The AF on the Canon EF 50mm f/1.2 I and EF 85mm f/1.2 II lenses are distinctly slower and less accurate than their slower cousins even on the top-of-the-line Canon 1DX.It has also been said the Fujinon 56mm f/1.2R isn't exactly a speed demon either.