I think that the term "best ever" is a bit over the top. The 35f2 is a REALLY good lens and I have not found anything to complain about with it. It is a big lens compared to the Leica 35's, but once it is on the camera, you forget it.
Most of my shooting is with medium speed black/white so resolution figures are of limited interest as most modern lenses will out resolve Tri X!
I like the whole line of Zeiss lenses as they have a remarkable consistency to their rendition. Very smooth, medium contrast and it does not matter if it is a 21 or a 50, the neg's look similar, only the angle of view changes.
At the moment I have been using the 21/4,5 ZM Biogon and, yes it has less distorion than any of the 2.8's (Zeiss/Leica) and it is a small compact lens. I dont know if it is "better" than the 21/2,8 ZM or the Leica 21/2,8 ASPH. The Zeiss line is interesting for one reason, it is a brand new lens line, designed from the ground up to perform as well as they can be made within the constrains of cost. The formulas used are tried and true - Zeiss in many ways are better lens designers than Leica and although Leica currently makes what I consider two of the best Rf lenses ever made, the 50f1.4 ASPH and the 75f2, Zeiss has a better program across the board!
The Planar 50/2 is in my estimation a better lens (for me) than the Summicron 50, the 25/2,8 ZM is a favourite of mine as it "looks" less wide than the 21 and the 28/2,8 ZM gets used occasionally, but in that focal length I prefer the miniscule 28/3,5 VC. The fact is that all of these lenses, by Zeiss, by Leica and by Voigtlander and others can do everything I want them to do and the brand does not really matter.
This said, one M2 is sitting on my table with the 35/2 ZM on it, the other one has a 50f1.5 ZM Sonnar on it and my R3M sports a Jupiter 85/2 - The Zi has the 21/4 VC-P lens. As long as I dont screw up, all of these will do a good job for me. We have a tendency to fret to much over specifications that are put out by manufacturers rather than just go out and shoot! I very much doubt that my shots are better because i use lens A instead of lens B based on optical charts. They might be better because lens B handles better in my hands instead!