I always considered the Summaron 35f2.8 the "secret" perfect 35. It is very good, even by todays standards. It is in many ways a better lens than the Summicron v1/2/3 - particularly for close and tight shots. Less flare prone too. OK, it does have an infernal Infinity lock - but that can be disabled.
The only contender for best medium speed 35 has to be the new Zeiss ZM 35f2.8 Biogon - and, yes, it is a slightly better performer (contrast/resolution etc) - but it is a bit bigger and will not accept 39 mm filters. My contention is that if you have a Summicron 35f2.8 or a Biogon 35f2.8 - you are all set for summer and slow, hi-res film.
Another benefit of the Summaron 35f2.8 is that it was mainly bought by "hobby" users back when - and they kept them in nice shape and used UV filters too. You have a far better chance of finding an optically clean 35f2.8 after 40+ years than a Summicron 35f2 v1 - or a Summilux 35f1.4 from the 60's. Pro's bought the fast lenses as they needed them - and usually did unspeakable things to them too (cleaning front elements by spitting on the lens and wiping it clean with a shirtsleeve, keeping it in a pocket with no caps and rarely, if ever using UV filters).
I still have a Summaron 35f2.8 - getting a bit ratty looking on the outside - but glass is clean ( stopped the spitting/shirtsleeve cleaning decades ago) - and it looks really nice on a M2 - and always deliver! Ok, I also have the new Biogon 35f2.8 and I do use that more - but the Summaron can usually holds its own.