The Leica 35mm f/2 APO-Summicron-SL ASPH is a spectacular performer with apparently zero or near-zero distortion, which makes it the best lens to have for the SL system since all its MTF goodness needs no sharpness-robbing distortion correction. Get yours today!
That said, it does have one very significant quality drawback—veiling flare, which I will document in all its horror. On that front it is quite disappointing.
In general, a 35mm focal length with no distortion and extreme performance is to me the holy grail of camera lenses, as I see 35mm as the very best all-around focal length.