I personally prefer the Zeiss Biogon T* 2,8/25 ZM. Less distortion and one of the best ZMs in the line up.
As for 21mm, there are a LOT of choices out there, and I honestly don't think you can go wrong with any of them. The 2,8/21 is nice, as is the 4.5,21 - but you'll probably want to use Cornerfix with the latter for best results. The CV 21mm f/4 Color Skopar is a real peach too. Super small, cheap and really quite good. If you insist on a Leica lens, the new Super Elmar-M 21mm f/3.4 ASPH is the way to go, IMO.
As for 21mm, there are a LOT of choices out there, and I honestly don't think you can go wrong with any of them. The 2,8/21 is nice, as is the 4.5,21 - but you'll probably want to use Cornerfix with the latter for best results. The CV 21mm f/4 Color Skopar is a real peach too. Super small, cheap and really quite good. If you insist on a Leica lens, the new Super Elmar-M 21mm f/3.4 ASPH is the way to go, IMO.