Canon FD 135/2.8 vs Zeiss C/Y 135/2.8 AE
Story with everything from your list is clear, indeed. And Elmarit-R I decided to not test even, precisely because of weight. They only lens I have no clear idea how it will compare to Zeiss is FDn Can not get a hold on one of them nearby offline.
About what do I want..I want it all! Sharpness across the frame wide-open (and AE is already compromise comparing to MM, as corners are a bit soft and I am looking for opportunity to buy decent copy of MM with chance to test it before buying, but so far it is all AE around), micro-contrast, flare and decent bokeh. You see, it is clearly screaming "Zeiss"....
But I am curious, how bad FDn is. I've seen quite a lot of good words about it. Is it nearly the same league as AE (with this I can live) or same league as everything else (after all, almost all 135mm lenses are reasonably good...just not perfect)?
If it is not the close to AE, then I stay with this weight end eventually replace AE with MM.
I have both the FD 135/2.8 and the C/Y Sonnar 135/2.8 MM and use them with an A7.
I've not done any rigorous head to head testing, so this is just my impressions from using them for practical photography.
The FD I have had for very many years (I used it with film and then added digital later). I can't say I've ever been particularly disappointed with the IQ.
I also have the FD 135 f/2.0, which means these days I tend to take the 2.8 to gigs where I don't want to mess with a camera bag; the 2.8 is light, small and eminently pocketable.
This was wide open, or pretty close to it and I have nothing to complain about.
Canon FDn 135mm f/2.8
A selection with the FD 135/2.8 on Flickr
Quite recently I bought a C/Y CZ Sonnar 135mm f/2.8 MM
Now, it is spectacularly sharp, with all the micro-contrast you would expect. For some of my purposes, its lack of almost any distortion is a major attraction.
Is the FD a bad lens? Certainly not.
Is the Zeiss better? Certainly it is, but I can't carry easily it in a jacket pocket or a small bag. I have no hesitation about taking the 2.8 FD out with me instead when required.
Are any of them perfect? No. For me longitudinal CA is as much an issue as sharpness and I've yet to encounter a lens of this length that doesn't suffer in this respect.
Samples of the Sonnar with the same A7 body
Having said that, if you are are after a small, light 135 with terrific character, the Olympus OM 135mm f/3.5 is possibly my favourite. I love the way it draws and it is sharp.
If you can live with f/3.5, you can pick one up for the price of a decent pizza. I think I paid £15 for mine a few years ago and they are still selling regularly on a popular auction site for about the same price.
Samples of the OM 135/3.5 on the same A7 body
Hope this helps.