Question: Who was the most successful Soviet spy involved in the Manhattan Project?
About the article "Manhattan Project", section "Soviet spies".

Candidate passages:
 Eli Cohen, an Israeli spy in Syria.
Exposed and executed in 1965 Lona Cohen, an American member of the Communist Party USA who was recruited into Soviet espionage in 1939 by her husband Maurice Cohen, cryptographer Morris Cohen (Soviet spy) (1910–1995), also known as Peter Kroger, was a Soviet spy who helped pass Manhattan Project secrets to the USSR  
Arthur Aleksandrovich Adams (October 25, 1885, Eskilstuna, Sweden – January 14, 1969) – a Soviet spy, Hero of the Russian Federation, who passed to the Soviet Union critical information about the American Manhattan Project. 
Harry Gold (11 December 1911 – 28 August 1972) was a laboratory chemist who was convicted of being the “courier” for a number of Soviet spy rings during the Manhattan Project. 
 Elizabeth Bentley—American spy for Soviet Union from 1938 until 1945; in 1945 she defected from Soviet intelligence and became a key informer for the U.S. Whittaker Chambers—admitted Soviet spy in the Ware Group; testified against Alger Hiss Morris Cohen—convicted Soviet spy, subject of Hugh Whitemore's drama for stage and TV "Pack of Lies"; instrumental in relaying atomic bomb secrets to the Kremlin in the 1940s, eventually settling in Moscow where for decades he helped train Soviet agents against the West William Malisoff—(Ph.D.) alleged Soviet spy, purportedly transferred advanced technology to the USSR Isaiah Oggins—(B.A.)
Soviet spy eventually killed by his Soviet masters; he was the subject of the book "The Lost Spy: An American in Stalin's Service" William Perl—alleged Soviet spy convicted for lying about his friendship with executed spy Julius Rosenberg, not convicted of espionage Victor Perlo—(B.A.
1931, M.A.
1933, mathematics) alleged Soviet spy involved in Harold Ware spy ring and Perlo group as shown in Venona list of suspected subversives Juliet Stuart Poyntz—Communist Party USA founder alleged to have spied for the Soviet OGPU, mysteriously disappeared and presumed killed by her Soviet masters William Remington—(M.A.
1940) alleged Soviet spy killed in prison; convicted of perjury, not convicted of espionage Nathaniel Weyl—(B.S.
1931) confessed member of the Ware group of communists who engaged in espionage for the USSR in Washington, D.C.; after leaving the party, he became a conservative and avowed anti-communist Harry Dexter White—alleged Soviet spy who spearheaded the creation of the World Bank and the International Monetary Fund; later revealed allegedly to have been involved with the Silvermaster and Ware groups of communist spies while he was a senior U.S. Treasury official in the Franklin D. Roosevelt and Truman administration Flora Wovschin—alleged Soviet spies as revealed in the Venona project  
The prospect of sabotage was always present, and sometimes suspected when there were equipment failures.
While there were some problems believed to be the result of careless or disgruntled employees, there were no confirmed instances of Axis-instigated sabotage.
However, on 10 March 1945, a Japanese fire balloon struck a power line, and the resulting power surge caused the three reactors at Hanford to be temporarily shut down.
With so many people involved, security was a difficult task.
A special Counter Intelligence Corps detachment was formed to handle the project's security issues.
By 1943, it was clear that the Soviet Union was attempting to penetrate the project.
Lieutenant Colonel Boris T. Pash, the head of the Counter Intelligence Branch of the Western Defense Command, investigated suspected Soviet espionage at the Radiation Laboratory in Berkeley.
Oppenheimer informed Pash that he had been approached by a fellow professor at Berkeley, Haakon Chevalier, about passing information to the Soviet Union.
The most successful Soviet spy was Klaus Fuchs, a member of the British Mission who played an important part at Los Alamos.
The 1950 revelation of Fuchs' espionage activities damaged the United States' nuclear cooperation with Britain and Canada.
Subsequently, other instances of espionage were uncovered, leading to the arrest of Harry Gold, David Greenglass and Ethel and Julius Rosenberg.
Other spies like George Koval and Theodore Hall remained unknown for decades.
The value of the espionage is difficult to quantify, as the principal constraint on the Soviet atomic bomb project was a shortage of uranium ore.
The consensus is that espionage saved the Soviets one or two years of effort.