What is the best mirrorless camera that is comparable in IQ to a medium format camera?
IMO, for landscape specifically, almost any recent camera with a 4/3 sensor or larger is roughly comparable to MF film.
16-20mp 43 sensors should be able to make excellent prints up to 16" x 20". At that size, high resolution 35mm sensors will give you a little more detail. The advantages of higher resolutions are more apparent with larger print sizes.
35mm sensors will have very good dynamic range, iirc around 14 stops. Smaller sensors have a little less. In my experience, that usually means you need to be more precise with exposure and/or work a little more in post with smaller sensors.
It also helps to know how much weight you're willing to carry; what's your budget; what's your usual output; what lenses you want to use.