This is the kind of comment I am after! For a long time I relied on Tamron SP glass because of price. But as soon as I tried Canon L glass, it was very solid and very pleasing. Currently I have all of 3 L-lenses.
Yet there are intangibles. I got the Voigtlander APO Lanthar 90mm F/3.5 SL-2, and I just love it! It is a joy to use. I think it has something like 270 degrees of smooth as silk focus ring. I'm not the greatest with manual focus, but I'd say my keeper ratio with this lens is pretty good. Maybe even 100%. The rendering is not nearly as good as my 70-200L. And though that is a zoom lens, I consider it to render better than any of my other lenses.
But in general, your words pull me pack a little from the Sigma Fever that is spreading. Has Sigma really slayed the Red Ring Kings? I have read some reviews state that one of the Sigma Art lenses was the best lens they have every used, bar none. Big words!
In the end it is not likely just about how sharp a lens is, but about your photographs with a lens .
Sigma has not really "slayed" the RRKs, or the pro Nikon glass, it is just that some of the new lenses are slightly better (note: some, and slightly) than the existing kings. F.e., the 12-24 FF doesn't look much better than the old EX version to my eyes, and I absolutely hated that. Also, speaking for canon, the 85L II is quite an ancient lens by today's standards .
The question really is whether you like the way a lens renders on your camera. In my case, Voigtländer only gets sharp stopped down quite a bit, and appears hazy to me from a tonal POV. Same with Zeiss, although slightly less so. My Canon lenses OTOH have great contrast, and it looks like that is one of the characteristics I like. Also, I find the sharpness roll-off to be not as abrupt as, f.e., Nikkors. That's another thing I like. What I don't like so much is that the newer lenes are getting bigger and heavier all the time , but that appears to be a necessity with higher MP sensors ....
BTW, rendering is one of the reasons why I have taken a big liking to MFT these days.
BTW2: If I had know what I know now, rather than acquiring a 350D back at the time when I did, I would have gone straight away for a 5D, which was available as well at that time, plus some L-lenses . I would have saved myself a lot of money . Eventually I went that route anyway.
Kind regards, Wim