Jim Kasson, who tests lenses quite carefully, has just posted test images in which the 50mm Z lens pretty clearly beats the Sigma Art 50mm, even with the Z wide open and the Sigma stopped down to f1.8. The Art is certainly one of the new breed of "Super 50s", which includes the Sony Zeiss 50 f1.4 and 55 f1.8, as well as the Canon RF 50 f 1.2, and perhaps others. These are all a different (and much sharper) animal from the classic double Gauss 50mm lenses (most if not all Canon and Nikon DSLR 50s, for example). The "Super 50s" tend to have 10 or more elements, instead of 6-8 for a conventional 50mm lens, and to use aspheric elements, low-dispersion elements or both, which aren't involved in classic 50mm designs.
Jim Kasson's test shows that the Nikkor is not only a "Super 50", but even in the upper echelon of that already refined class - the Sigma is an excellent and well-respected lens that doesn't give up much, if anything, to any of the others. Since there's no way to get the most obvious "Super 50's" other than the Sigma onto the Z7 body (Sony to Z and Canon RF to Z adapters don't exist), a direct comparison with the Sony or Canon lenses is impossible... There is very little evidence that any of them are in a whole different class from the Sigma, though - one or more might be as good as the Z Nikkor (the Sony Zeiss 55mm f1.4 and the Canon RF 50mm f1.2 are likely candidates), but almost certainly none will outclass it (the Canon RF is a full stop faster, and most of the others are a half stop faster).
That leaves one ~50 mm lens that might truly be better than the little Nikkor - the mighty Zeiss Otus 55mm f1.4. At 2.5x the weight (before adding the FTZ adapter) and nearly 7x the price, the Otus would have no reason to exist if it can't beat all comers! We have an interesting situation where each FF mirrorless system has a great 50mm lens that is probably preferable to any 50mm DSLR lens except the Otus. There doesn't appear to be much to choose between the mirrorless "Super 50s" across systems - pick the Nikkor Z, the Sony Zeiss or the Canon RF by what fits your camera, but 50mm belongs to mirrorless...
