The Voigtlander 12mm f/5.6 Ultra-Wide Heliar is the widest lens available in Leica M-mount. Despite having an ultra-wide angle of view of 121°, it offers a minimal amount of distortion. The lens has rangefinder-coupled focus to as close as 0.7m, and scale-focusing down to 0.5m. It’s built-in hood helps to keep flare down, but it still takes a 67mm filter if needed.
As far as image quality is concerned, the Voigtlander 12mm does suffer from vignetteing and color shifts in the corners, so be aware of this on the front end. Nevertheless, corner sharpness is quite good for such a wide lens, even at f5.6.