Summilux-R 1.4/50
There are two versions of this lens. Type 1 is very similar to the pre-ASPH Lux-M.
The last version performs as good or better than the Summicron-R.
I love the smoothness of the tonality and bokeh you get with the type 1. A truly great lens in b/w. Salgado shot a lot with this lens. There is a book of children's portraits that he did and I think a lot of those shots were made with this lens. He could have taken those shots with a coke bottle and they would still be brilliant, but I've seen some of them as real prints and they are truly beautiful.
The only problem is that of all the R bodies I really only like the R8/R9 and they are somewhat bulky for daily use (and my R8 sucks down batteries at an alarming rate)
But given that I mostly shoot Nikon SLR bodies I would have to say it's a toss up between the Zeiss ZF 1.4/50 and the old Nikkor-H.C 2/50. I shoot the Zeiss in low light, but the Nikkor during the day, because I like it's smoother, vintage look in sunlight.
But if I had to choose one of the two it would be the Zeiss, which I think is the best 50 out there for Nikon. Overall I rate the Zeiss only second to the Summilux.