Final answer:
The principle where the best property's value is pulled down by lower-valued properties is known as regression.
Step-by-step explanation:
The principle that states that the value of the best property in a neighborhood will be adversely affected downward by the value of many lesser properties is known as regression. This phenomenon occurs due to the impact that the overall quality of a neighborhood has on individual property values. While gentrification often refers to the upgrading of neighborhood through an influx of wealthier residents, regression reflects the opposite scenario where numerous lower-valued properties pull down the value of higher-end ones.