The phrase "most built up" can be replaced by several synonyms that convey a similar meaning. One such synonym is "highly developed," which refers to an area that has undergone significant construction and urbanization. Another synonym for "most built up" is "urbanized," which describes an area that has been transformed into a city or urban center. "Densely populated" is another phrase that can be used to describe an area that is heavily developed and has a high concentration of people. Lastly, "metropolitan" is a term that can be used to refer to an area that is heavily populated and has a high level of economic activity and urbanization.