Who is the most responsible for breaking the Enigma code?

The individual most responsible is probably the English mathematician Alan Turing. But without the early Enigma machine captured by French and polish spies it may not have been possible to build the machines necessary to complete the effort.
An early Enigma was reconstructed by the Polish General Staff's Cipher Bureau in December 1932-with the aid of French-supplied intelligence material that had been obtained from a German spy. Shortly before the outbreak of World War II, the Polish Cipher Bureau initiated the French and British into its Enigma-breaking techniques and technology at a conference held in Warsaw.