Juan Pujol García, codenamed GARBO by the British and ALARIC by the Germans, may be the most remarkable spy in history. A Spanish civilian who hated both fascism and communism, he approached British intelligence offering to spy for them. When rejected, he offered his services to the Germans instead—planning to be a double agent from the start.
Using a tourist guide, a railway timetable, and sheer imagination, Pujol invented an entire network of sub-agents supposedly operating across Britain. He sent the Abwehr increasingly elaborate (and entirely fabricated) reports from Lisbon, creating 27 fictional sub-agents including a Welsh nationalist, a Gibraltarian waiter, and a Venezuelan student. The Germans were so impressed they paid him an equivalent of $340,000 for his "network's" expenses.
When MI5 finally identified him and brought him to London, he became the crown jewel of the Double Cross system. His most crucial contribution came on D-Day: he convinced the Germans that Normandy was a feint and the real invasion would hit Calais. The Germans awarded him the Iron Cross; the British gave him an MBE. He faked his own death in 1949 and lived quietly in Venezuela until his discovery in 1984.