When it comes to finding synonyms for "most horizontally", there are a few different options to consider. One possible choice is "flattest", which emphasizes the lack of inclination or slope in a given object or surface. Another option is "level", which can also refer to a lack of variation in height or angle. Similarly, "even" can be used to describe a surface or object that is consistently flat and uniform. "Horizontal", the root word of the original phrase, can also work as a synonym. Finally, "planar" is a more technical term that refers to a flat surface or plane.