The obvious choice is probably the 28mm f2.8 SE, cheap, small, lightweight and very good but maybe not great, optical quality. However, with the bonus that it matches the retro look of the ZF
I went for the much more expensive 26mm f2.8 (to go with my ZF and 40mm f2 SE). More compact than the 28mm f2.8 SE, and with excellent performance in the centre, but not that impressive as you move out to the edges. Part of this is distortion, and can be improved by focussing in front of infinity, assuming distant landscape photography
Depending on what you photograph, the edge performance may not matter, and many people seem to rave about how good the 26mm is ("almost an S-Line lens"), The formal reviews seem to match my experience.
With Capture One, I can largely correct the edge sharpness fall off (most software only corrects for distortion and vignetting, but not sharpness), so I can live with it for my use, and I like the convenience of its pancake shape and its focal length. DXO also corrects for sharpness fall-off (automatically), but C1 makes a better job of it.
Adding to why I chose the 26mm, is that I also use it on my Z8, where I can crop to APS-C (by pressing the video record button) and get the equivalent of a 26mm (45Mp) and 40mm (20Mp) without changing lenses. Also, I can use it on a Z50II giving me a very compact camera setup with the equivalent of around a 40mm angle of view on 20Mp. I like the 40mm focal length as my "standard" lens, and I have found the 26mm to be a nice compromise between a 24 and 28. I've had both in the past.
The edge performance on the 28mm f2.8 SE is meant to be better than the 26mm, and if I were primarily concerned with using the lens for wide vista landscapes, I would find it difficult to choose between the 28 and 26.