I my opinion this would be Operation Barmaid.

Read my answer to the question originally asked and answered on Quora here.

The Untold Story of Britain’s Most Famous Submarine: Amazon.co.uk: Stuart Prebble: 8601404603235: Books

HMS Conqueror is Britain’s most famous submarine. It is the only sub since World War Two to have sunk an enemy ship. Conqueror’s sinking of the Argentine cruiser Belgranomade inevitable an all-out war over the future of the Falkland Islands