Sigma is a maker of enthusiast level (at best) lenses. I bought a couple, many moons ago, before I knew better. Replaced with Nikkor glass, which, while superior, is still inferior to the best: Fuji, Zeiss, Schneider, Rodenstock, and Leitz (for Leica).
Zeiss and fuji are generally the best, if you are buying new.
With all the adapters available, a 60 year old Schneider Xenon, or Zeiss Planar will blow any Sigma lens ever made out of the water, at less than half the cost of a new Sigma.
Fuji now manufactures the Hasselblad, AND, more importantly, the glass for Hasselblad.
Why? Planars (Xenon is Schneider's planar design) are sharp even at maximum aperture, across the image plane.
This is why they cost more, and are among the best.