Very interesting. My verdict:
To me, all are good, and Ketron is the most realistic one. Casio doesn't fully convince me especially in the low keys, it sounds a bit flat (but still quite good). The Rolands and the Clavia sound nearly 100% realistic to me, but Ketron still beats them in realism. BUT (!) the Ketron sample has quite a lot of natural piano reverb in its sample, which makes it sound a bit indirect and, what's the right word, strident, sharp, shrill. I experienced the same with the main piano sample of my former Ketron Klavipro. In that respect I like the more direct and dry sound of the Rolands or Clavia more (depending on the type of music played), even if not so outstandingly realistic.