After trying a number of different 50mm Nikkors I settled on the the AI 50/1.8 as having the best bokeh (and good resolution too). It has a 7-blade aperture so out of focus highlights in the background or foreground have a more organic 7-side shape. The background bokeh is also smoother without double-line blurs, although it gets worse towards the edges of the image. I never tried the 58/1.2 Noct but I gather that has good bokeh also.
Yes, the 58/1.2 has a nice bokeh too, not just wide open. But the best in this department may be the Zeiss 50/2 (Milvus - much better coatings), and in all others as well. Only drawback was its size and weight. On the Df it used to be my top choice, then came the Summicron R 50/2 (alas, with outdated coatings, hence high flare potential) ; rendering is more subtle than the Zeiss; a classy lens. My third choice was the 50/2 AI, imperfect, but an "it" lens; its recessed front lens is a bonus outdoors. Then, the 50/1.2 AI which I used for street shots, esp. demonstrations and similar, don't ask me why.
For some reason I only recently got a decent copy of the 50/1.8 AI (together with an FE), but I did not use it extensively.
