Nikon 50mm f1.2 Ai-S or Sigma 50mm f1.4 - It's hard to tell one apart from the other.... especially if you use either at f1.4 or stopped down at f2.0. What would give the Nikon the edge is that when shot using the same conditions / camera settings the Nikon comes off with a bit better saturation / color / warm. Of the list, the Nikon 50mm f1.2 would be the preferred option for me.
If you don't mind spending a little more, you could get the Zeiss 50mm f2.0 Macro and get some really stunning images with typical "zeiss-ness" - something neither the Nikon or the Sigma can offer.
Or if you would prefer to go another route for optimum image quality wide open,
Option 1: The Olympus OM 50mm f1.2 can now be converted to the Nikon F-mount using the Leitax Adapter. Not a bad deal for one of the best 50mm lenses I have ever used.
Option 2: Leica Summicron 50mm f2.0: Amazing wide open at f2.0 with impressive sharpness and a wonderful bookeh. I seldom use this lens stopped down and very rarely does it make it to f2.8
