it depends on how you look at it really, and on individual lens comparisons.
as a general rule, my experience with Nikon lenses which is borne out by looking at Photodo's MTFs is that they have/had a lot of coarse structure contrast but field curvature once you started getting into finer detail + a lot of divergence of sagital and tangential MTF values ie astigmatism which leads to the impression of blurring of fine details. in addition, I personally find the feel of their lenses to be bottom shelf, while the early metal ring Rokkor 58/1.2 I had was probably the second finest built lens Ive ever owned (my Leica-R 60/2.8 takes first place).
the Japanese lens makers were all more or less in the same place, despite what internet rumor has to say. they made different compromises which led to most of the differences, and some designs proved superior to other contemporary lenses by a significant margin (e.g. the Canon 55/1.2 ASPHERICAL, Nikon 200 ED) before the others caught up.
If you want a hard answer, the latest generation of R lenses made by Leica or the Contax lenses made by Kyocera under the direction of Carl Zeiss are in my opinion a step above in the meaningful measures of image quality (fine detail reproduction, flatness of field, low astigmatism). then canon and olympus. then minolta and pentax. then nikon. just my 2c.