I've tried many wide angle lenses for may formats and systems. Leica has sold a couple of near zero distortion lenses; the Super Angulons are the most commonly seen. The newer 21's have more distortion. The older, slower lenses all had near zero distortion, while the faster lenses and the 28 and wider lenses produced after the M5 came out have more.
The V/C 12, 15 and 21 have a little bit of distortion, but it's rarely a problem. The old Leica Hologon had none, but it's almost perfectly symmetrical, or as symmetrical as a lens designed to focus at long distances can get. Unfortunately at f/8 or t/16 with the filter it's a bit hard to use at times. It also doesn't really focus to infinity, and it's too valuable for me to take a file to it.
Until recently, really good distortion free or nearly free strongly retrofocus lenses were impossible or impractical to design. That's no longer the case; just look at the new 24 and 17TS-E lenses from Canon. Outstanding in every way, including distortion and with gigantic image circles. I had despaired that Canon could design a decent lens wider than 35, but these proved me wrong. If anything, they show up recent Nikon designs.
Nikon has had a spotty record in retrofocus wides. Most of their 20's have had fairly low levels of distortion, as did the 15/3.5 and even the f/5.6 wasn't that bad, but the 18's were/are among the very worst. Utter disasters re distortion. As is, unfortunately, the otherwise excellent 21/2.8 Biogon for SLR's. It's not as bad as the 18's of Nikon but the distortion levels make the lens quite unattractive for me otherwise.
As mentioned, many zooms have serious distortion problems but there are exceptions. A lens that I got as a replacement/extension for the 16-35/2.8 Canon (quite a bad lens in many ways considering the price) is the Sigma 12-24 for full frame; the widest zoom and very little distortion! It's not nearly as good as the 14-24 Nikon or the 7-14 Panasonic, but as far as distortion performance goes it's excellent. The Panasonic, by the way has extremely high levels of barrel distortion, but due to the software correction it's unlikely one will ever see any distortion in normal use. In practice it has extremely low levels of distortion. This is in my opinion an excellent way to use technology to advantage. Through letting the distortion go where it may and correcting it in software Panasonic has been able to correct most of the other aberrations to an extremely high level and produce and amazingly small and first rate lens that is competitive with anyone's.
In any case, for rangefinders the older, slow lenses are generally excellent regarding distortion, and the newer ones that allow you to TTL meter are not as good. I'm keeping my Super Angulon f/3.4, even though I have the 21 Summilux, the 21VC and WATE. Each has it's purpose.
