Let us know what you think of the samyang/rokinon 12mm. Considering the micro 4/3 version of it (24mm equivalent).
One of the other options for me is the Olympus 12mm prime but it's more than twice the cost of the samyang/rokinon. I figure for the intended purpose (landscape) where you'll be shooting at infinite focus a lot of the time anyway, the lack of auto focus is a very minor inconvenience and not worth 2x the cost.
The trend has certainly been to ultra wide angles for landscapes last few years but it hasn't always been that way. Even a 35mm or nifty 50, whatever equivalent that is in your particular crop sensor will produce stunning shots. The real key, in mu opinion, to a good landscape lense is great sharpness corner to corner even with the aperture wide open.