My 2¢ if I may...
As my member-name suggests....I've used Nikkors for 50 years now, and it wasn't until I went digital that I used any other brand (I shoot with Canon dSLR bods, so I have a few Canon lenses)....I used Nikkors because all the PJs I worked with did...simple...and still use them on my Canons because thay work as well with the Canon as I do...
....and I may just have been off the main lines of techology and gear talk but I don't recall any discussions about subtlies such as "bokeh" and lens "character" back when I was a PJ....
But it's just such subtlies that I read about now...bokeh, 3D effect, color rendition, contrast and such...and I can see the difference, and it may be THOSE differences that one poster might be referring to when he recalled that people COULD tell the Leica lenses apart from others...It wasn't specified so I'm just guessing here...some lens's bokeh is "busy", jagged and distracting AND TO COMPLICATE MATTERS IT HAPPENS ONLY AT SPECIFIC F-STOPS...
....and so one could search out specific lenses for specific characteristics, and it would be an interesting aspect of one's style and technique that makes one's photography distinct....whether ALL lenses by ANY ONE manufacturer would conform to that 'rendering' would be an interesting technical experiment and test to conduct...but even if so, that is hardly a defense of OP's question about quality as a quantitative measure as upposed to quality defined as the "nature" and "character" of a lens...
Just one example of quality as a quantitative measure rather than as a set of characterisitcs...one can make the case that 50mm F2 lenses are (generally) better than F1.4 lenses because they are more evenly sharp from edge to edge than are 50mm F1.4 lenses....does that make an F2 lens "better?"....well not at 1.4

.....and by F2.8, F1.4 lenses often match the "quality" (quantitatively speaking) of the F2....(and of course F1.2 lenses beat them both at 1.2 even if that lens is soft as clouds towards the edges, even at mid-apertures!)
Another point to be made is how a lens functions....a lens whose focusing barrel that has a shorter rotational range from MFD to infinity than another lens may be easier, or more difficult to use, depending on the USER....A videographer would prefer the more gradual focusing lens, as do many still photogs...others prefer the shorter "throw" because the point of focus pops in and out more readily and so is easier to judge when focus is "spot on".
Any such discussion should take into account these, and other considerations when "judging" one lens against another...
I'm guilty myself, having sold measurably EXCELLENT lenses to take advantage of another lens's "quality" (characteristics)...sometimes as I've measured myself...for example...The 85/1.8 Nikkor-K I had was 1/3 F-stop "brighter" at 1.8 than the 85/1.8 Nikkor-H I also had at the time, so I kept the "K", even though I had had GREAT results with the "H"...and don't think I sometimes wonder, (with just a twinge of regret), whether that was reason enough to "upgrade" from a lens whose excellence was proven by my experience....oh well...
Another "downgrade" I made was going from Canon's 50mm F1.8 MKII (an optically VERY good lens) to the 50mm F1.8 MK I because the wobbly front focusing barrel of the MKII often wobbled juuuuuuust a bit out of focus when I was shooting @ 1.8 whereas the 20-year older MK I version's interior focusing barrel was "tight" and held focus better... but that's part of a problem of AF design, not optics....still, it is that kind of consideration (not to mention that the older version is better built) that one should make when buying lenses...
Finally...I know, I know this is getting a bit long...still....the SIZE and weight of a lens as it fits on the body makes a difference in how well any one person can use it and hold it steadily...often bigger lenses "feel" better on bigger bodies than smaller lenses but that is not a measurable quality as much as quality of preference...
Okay...maybe more than 2¢....hope it was worth it....to someone