I've had the 50 f/1.8D and 50 f/1.4D Nikkors. Have tried the 50 f/1.4G but never tried the Sigma. From my experience I will rank them like this in order of IQ and bokeh:
1. 50 f/1.4G
2. 50 f/1.4D
3. 50 f/1.8D
But I still like my Voigtlander Nokton 58mm f.1,4 SL II best. The Sigma looks like very good competition with the Voigtlander but goes for twice as much as I've paid for the Voigtlander.

. No back/front focusing issues too since I rely on my eyes instead of AF.