I doubt that I said it was the best 35 Summicron. I like the 35 version 1, but only for flashbacks to the 60's. By todays standard it is not the sharpest, not the highest resolving one either. However, coupled with tri X and D76 - Man, what a trip!
The version 2 with the tabbed aperure control is actually a very good lens and though the tab takes a while to get used to - it is quite a good performer. Some people feels it is not as good as the Version III, but I have never seen that much difference.
I like the version III personally. I kept knocking the tab loose on my Version 2 and between them I cant see much difference, if any between the II and III.
The version IV is one of those lenses that has gotten some weird "mystical" quality to it. It is good overall, but wide open performance is just middle of the road today. The bokeh, or "fuzzy stuff" is less harsh than the Asph Summicron, but as to why they are so expensive, I dont know, but then who can explain Leica pricing!
Mechanical quality is better on the early ones! The version IV can suffer from aperture rings failing and also from the front element pack coming loose (not falling off, just rattling a bit). The "no plastic part" version III
feels more solid.
If you are looking for a "shooter" - look out for a version III (and get the hood with it) or even consider the VC 35f2.5II or, if you can handle the size, the ZM 35f2.0. I havent had much experience with the UC Hexanon, but the few rolls I shot with one looked fine.
You will have trouble finding a bad 35 for a M-mount. Any manufacturer knows that this is one of the prime lenses for Rf's and is going to make sure that whatever they make is going to be as good as the price allows for.