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The 21 is definitely up there. Having owned the 17-40L, I can say that if there is enough copy-to-copy variation to account for the difference in image quality that I have seen between them, then a new record has been set, by orders of magnitude.
I disagree with the 50MP. It is a great lens, but it has some distinct flaws which make me like it less than the more modest 50P. The 35/1.4 is also up there, really great lens with beautiful rendering. The 85P is also great, similar to the 50P, but neither could probably be said to be the greatest Zeiss lens for Nikon.
Depending on what you like, I am guess it would be a battle between the 15, 21, 35/1.4, 100MP and 135/2. I don't own the first and last of these, hence the guesswork.
The ultimate Zeiss lens for Nikon will be the 55/1.4