The phrase "public enemy number one" is often used to describe an individual or group who is considered a serious threat to society. However, there are several synonyms that can be used to convey the same message. For instance, one could describe someone as a "menace," a "villain," a "pariah," a "rogue," or a "terror." Other possible synonyms include "scoundrel," "outlaw," or "criminal mastermind." Each of these terms conveys a sense of danger and threat to the public, and they can be used interchangeably with "public enemy number one" in many cases.