I have (or had) most 21's available in Leica mount, and currently have the 21/2.8 ASPH, 21/1.4, 21/3.4 SA, 16-21TE amd 21/4 Cosina. I've also used the 21/2.8 and f/4.5 Zeiss for a while, and had the 21/1.8 Cosina for a week a couple of months ago. A local friend has good connections with Kobayashi-san and got serial no. 00001. So it might not be quite representative, but it shouldn't be far off either. Mechanically it felt very good; probably the best of any Cosina lens I've had or tried so far.
On the basis of the sample I tried, however, it would be the one I would get at this point if I could have only one, unless it was really too big. It's getting close to, but not quite as big as the 21/1.4, and it's lighter. It is as sharp as the Summilux with no more optical problems, and it has noticeably less vignetting. I would definitely get it in preference to the 21/2.8 Zeiss or 21/2.8 ASPH, let alone the pre-ASPH. Currently the best 21 is the SE f/3.4, but it comes at the cost of the lack of speed... and relative cost. In use you'd likely not see the difference in quality between it and the 21/1.8 Cosina, though, just as most of the others are also at the same level. The most obvious and easily seen difference is in the level of distortion, where the faster lenses do somewhat (but not drastically) worse.
If you want small, then there is really only one workable choice: the 21/4 Cosina M-mount.
A number of years ago I did a fairly comprehensive test of four 21's for a magazine, which besides the 21/3.4 SA, 21/2.8 ASPH and 21/4 Cosina included the Ricoh 21/3.5 (not that great, and very rare) and the Cosina wasn't that far behind the 21/2.8 ASPH. I also tested some other 21's that are less common, but they didn't really rise above the Ricoh. The 21-35 Hexanon is interesting, but now it too is more valuable to collectors than shooters. If the price and availability had stayed reasonable it might have stayed an acceptable choice as the versatility and performance were quite decent.
I'm not getting rid of the Summilux, because it is really 2/3 stop faster, and the pain of paying for it has been left behind. The TE stays for obvious reasons, the SA is my long time love for film and the Cosina is tiny. The 21/2.8 ASPH will have to go.
Henning