Story: When I enter a new statement with a quantity with a unit I want the selectable units to be ranked meaningfully so it is easier to find and select the desired unit.
Alternative story (proposed by @Jan_Dittrich): When I enter a quantity, I want the selectable unit to be a meaningful match with the statement. E.g. if the statement is "height", I like to have units of distances.
Units should be ranked according to how often they are used as units across Wikidata.
This is the first step and could be good enough for a good user experience.