The answer depends on what you mean by “KGB spy.” If the question is about agents run by the KGB, the most famous is Kim Philby — the British MI6 officer who spied for Moscow for nearly thirty years and whose betrayal compromised hundreds of operations. If the question is about KGB officers themselves, the most famous are Yuri Andropov (chairman 1967–1982, later General Secretary), Oleg Kalugin (the senior PGU officer who later became a sharp public critic of the service), and Oleg Gordievsky (the most senior KGB officer ever to spy for Britain). For more on the KGB article; the Cambridge network that produced Philby is detailed in the famous-spies article.
The distinction matters, because the KGB’s most spectacular victories were not its own officers. They were the citizens of the adversary who were recruited, often in their student days, and who rose to positions of trust inside Western governments and scientific institutions. Philby is the case that historians most often cite — both for the duration of his service and for the volume of secrets he passed. The Cambridge network he was part of delivered Anglo-American policy in the late 1940s to Moscow at a level of detail that MI6 did not believe was possible until 1995 and the Venona decrypts.
The short list of candidates
A defensible list of “most famous” candidates, with their actual contribution, looks like this:
- Kim Philby (1912–1988). British MI6 officer. Recruited by the NKVD in 1934. Head of the Iberian section during the war; MI6’s Washington liaison in 1949–1951. Warned Maclean and Burgess of their impending arrest in 1951. Defected to Moscow in January 1963, after years of being watched. Received the Order of the Red Banner of Labour. His memoir, My Silent War (1968), is one of the most-cited primary sources on the Soviet side of the trade.
- Yuri Andropov (1914–1984). Chairman of the KGB from 1967 to 1982; General Secretary of the Soviet Union from November 1982 to February 1984. The only KGB chief to become Soviet leader. The agency under his chairmanship signed the orders to expel Alexander Solzhenitsyn in 1974 and to intern dissidents in psychiatric hospitals. He is the only one on this list whose career is primarily inside the KGB.
- Oleg Kalugin (b. 1934). Major general in the KGB; head of the agency’s foreign counterintelligence directorate, Directorate K, in the 1970s and early 1980s. In 1990, he became a public critic of the service; in 2002, he was convicted in absentia in Russia of treason. His memoir, The First Directorate (1994), is an insider account of PGU operations in the United States.
- Oleg Gordievsky (b. 1938). The most senior KGB officer ever to spy for MI6. Recruited in Copenhagen in 1974 after a long courtship that began in 1968. He served as rezident in London in 1985 and was extracted to Britain in 1985 after a Soviet counter-intelligence operation nearly caught him. His debriefings, in particular his recollection of the Politburo’s thinking on the Chernobyl accident of April 1986, were the basis for a great deal of Western understanding of the late Soviet leadership.
- Aldrich Ames (b. 1941). Not a KGB officer but a CIA officer who was run by the KGB. He volunteered to the Soviet rezidentura in Washington in April 1985 and was paid $2.7 million for betraying at least ten CIA assets, who were subsequently executed. He is the most damaging American traitor in the second half of the Cold War. See the separate entry on the famous Cold War spies.
- Klaus Fuchs (1911–1988). A German-born British physicist who worked at Los Alamos and passed the design of the implosion bomb to the Soviet Union from 1944 to 1946. He is the most consequential nuclear spy in history: he gave Moscow the design that produced the first Soviet atomic bomb, RDS-1, in August 1949. He was uncovered via Venona in 1949 and confessed in January 1950. He is covered in the [Manhattan Project espionage articles entry].
The historiography on this question is dominated by the British side — partly because British intelligence has opened its files (most recently in Christopher Andrew’s authorized MI5 history of 2010) more than either the American or the Russian, and partly because the Cambridge Five case has been, since the 1950s, the most commercially interesting of the Soviet penetration stories. The Russian side — the Soviet view of Philby, the actual scale of the PGU’s successes, the post-1991 memoirs of the Kalugin and Gordievsky generation — is now also a substantial literature, and it is the place to look for the most candid answer to “most famous.”