it is difficult to pinpoint a single threat actor as the most notorious, as the threat landscape is constantly evolving, and new actors emerge over time.

1. Advanced Persistent Threat (APT) Groups: These are sophisticated and well-funded threat actors, often associated with nation-states, conducting long-term cyber espionage and targeted attacks. Some notorious APT groups include APT29 (Cozy Bear), APT28 (Fancy Bear), APT32 (OceanLotus), and others.

2. Lazarus Group: This is a North Korean state-sponsored hacking group responsible for high-profile attacks, such as the Sony Pictures hack and various cyber espionage campaigns.

3. Russian Cybercriminals: Russia has been associated with several high-profile cybercriminal groups, like the Carbanak group and the Evil Corp group. They have been involved in large-scale financial cybercrime and ransomware attacks.

4. Chinese Cyber Espionage Groups: China has been linked to various cyber espionage campaigns, with groups like APT1 (also known as Comment Crew) and APT10 (Stone Panda) being well-known examples.

5. Iranian Threat Actors: Iran has been involved in cyberattacks against various targets, and groups like APT33 (Elfin) and APT34 (OilRig) have gained attention for their activities.

6. Ransomware Groups: Several ransomware groups have caused significant damage globally, demanding ransoms to decrypt victims' data. Notable groups include REvil, DarkSide, Maze, and Ryuk.

7. Cyber Mercenaries and Criminal Syndicates: There are also smaller, lesser-known groups and individuals who engage in cyberattacks for profit or on behalf of third parties.

Keep in mind that the threat landscape is ever-changing, and new actors may have emerged or existing groups may have shifted their tactics and focus. To stay up-to-date on the most notorious threat actors, it's essential to follow reliable sources in the cybersecurity industry and government cybersecurity agencies' reports.