Nikon 50mm Lenses

Nikon 50mm Lenses

At the time of writing there are four Nikon 50mm's. 2 x D lenses and 2 x G lenses. Mix in different 'f-stops' of f/1.4 and f/1.8 and you might be confused. Which of these 4 Nikon 50mm lenses is the one that should buy?