I am a bit undecided. In the end I would like to have a 21mm lens.
However the Canon 20mm seems to not be that great. Everybody raves about the Zeiss. But it is MF. On the other side I can see how for such a wide lens one can maybe do a lot with hyperfocal/estimate distance focusing.
The option would be a 24/1.4 (or just use my 24-70) but I think overall I would prefer 21mm.
If a new 16-35 came out this could be an option as well.
The Zeiss 18mm is a little too wide for my taste.