I agree with Björn about the 105/2.5.
Others ? well, the 50/1.2 maybe (slight preference for the AI version, because of lesser vignetting - the AIS "cuts corners"). The kit 50/1.8G for sure.
I definitely like the 28/2 on Df, especially for B&W works.
Nikkors aside, the Voigtländer 40/2 and 58/1.4 are a delight to use, the 40/2 probably being the most convincing one. It is very good on 16MP, with perfect size and ergonomics. It competes with the Zeiss 35/2, which is the better lens, with better contrast, sharpness and king of against-the-light shots (very good at night too). But the small size and weight of the Voigtländer, and its assorted close-up lens, are particularly enjoyable.
But on the Zeiss side, the 50/2 macro is my most used 50mm lens on Df, and made the trip to Iceland with the 105/2.4 (and the 24/3.5 PC-E). Glaciers, waterfalls, geysirs, everything.
My daughter is also eyeing on the Df, and would select the 40/2 and 105/2.5. It is also my current walkaround combo. If limited to one lens, then the Zeiss 50/2 would be the choice.