Reading up on this it appears to me many of Germany's spy operations inside the UK were pretty quickly shutdown. I investigated a half dozen spies who were captured by the end of 1942.
- Dorthy O'Grady arrested December 1940
- George Johnson Armstrong executed July 1941
- Norah Briscoe arrested March 1941
- Theodore_Schurch captured June 1942, executed Jan 1946
- Jose Estelle Key executed July 1942
- Duncan Scott-Ford executed Nov 1942
- Mathilde Carré - Arrested by MI5 in 1942.
Other's I was considering who were not arrested during the war...
- Horst Kopkow - An SS Officer, responsible for counter-sabotage and counterespionage. During the war, Kopkow's agents captured several hundred Soviet and British agents. Kopkow was informed and consulted over every capture, although he never left his headquarters in Berlin. One of his major efforts was the destruction of Red Orchestra and Rote Drei espionage networks. Security police also captured agents of MI6 and SOE. Kopkow authorized several hundred orders to execute the agents. This continued to the end of the war in 1945. His superiors rewarded him with medals.
- Harold Cole - A Brit, who worked with the French Resistance. Turned over about 30 pages of notes on the F.R. to German Military Intelligence. "Described as the worst traitor of the war". Was active throughout the war.
- Elyesa Bazna(Cicero) - Employed as a valet to the British ambassador to neutral Turkey from 1939. Passed on important information about many of the Allied leaders' conferences, including the Moscow, Tehran and Cairo Conferences. The details for the Tehran Conference were important for Operation Long Jump, the unsuccessful plot to kill Franklin D. Roosevelt, Joseph Stalin, and Winston Churchill. He had also conveyed a document that carried the highest security restriction (BIGOT list) about Operation Overlord (the code name for the Invasion of Normandy in June 1944). Stopped selling information to the Germans by the end of February 1944 and left the embassy within a month or so. After the war, Bazna was questioned for war crimes, but he was never charged with espionage.
Other than Elyesa Bazna who stopped selling information early 1944, these candidates seem to be pretty low level. More tactical than strategically damaging. They all did damage, but I was wondering. Who was the Greatest German Spy of WWII? To make this more specific I'll limit it to contemporary WWII German perspective. Is their a consensus among historians?
Per Denis de Bernardy suggestion, looking for someone who reported intelligence which reached Nazi Germany and not simple a high ranking official in the Nazi intelligence. Most successful German spy means successful in collecting information valued by German Intelligence, not valued just due to his rank and leadership.