Submarines in a walk. Nothing else comes even close.
Had the KM had a reasonable number of boats (the total force was only 57 boats, only 23 were capable of operation into the Atlantic shipping lanes) the Reich could have done vastly more damage. The Reich would not have been able to force a British Surrender, but the issue would have been very close. The U-boats sank 14 MILLION tons of shipping. Still, the KM failed, mainly because the Allies, primarily the U.S., constructed 38 million tons of merchant tonnage, built hundreds of specialized escorts. broke the Reich's codes, and invented a type of warfare that had never before been attempted.
In the Pacific the result was dramatically different. The USN did starve Japan while sinking three of every five merchant ships, of any size, that the Japanese operated during the entire war.
The second best weapon was mining, both submarine deployed and air deployed. the U.S. mining campaign effectively made the Japanese Inland Sea a "no go" zone and made the waters between Japan and Korea a death trap.
Aircraft are a nice addition, especially if you can get them into restricted waters that the enemy has to transit, but they are far less capable in the open sea (too much sea, too few aircraft, limited operational window) and are much more vulnerable to the defensive armament that can reasonably be added to a merchant vessel without compromising its primary function. Among aircraft weapons the best option, by far, is the air dropped torpedo. Unlike warships, which have the speed and maneuverability to often avoid the deployed weapon, merchant ships (in WW II generally possessed with top speeds of 10-12 knots) are unable to dodge a torpedo. One torpedo was sufficient to sink or cripple almost any merchant vessel used by any nation throughout the war.
Had the KM had a reasonable number of boats (the total force was only 57 boats, only 23 were capable of operation into the Atlantic shipping lanes) the Reich could have done vastly more damage. The Reich would not have been able to force a British Surrender, but the issue would have been very close. The U-boats sank 14 MILLION tons of shipping. Still, the KM failed, mainly because the Allies, primarily the U.S., constructed 38 million tons of merchant tonnage, built hundreds of specialized escorts. broke the Reich's codes, and invented a type of warfare that had never before been attempted.
In the Pacific the result was dramatically different. The USN did starve Japan while sinking three of every five merchant ships, of any size, that the Japanese operated during the entire war.
The second best weapon was mining, both submarine deployed and air deployed. the U.S. mining campaign effectively made the Japanese Inland Sea a "no go" zone and made the waters between Japan and Korea a death trap.
Aircraft are a nice addition, especially if you can get them into restricted waters that the enemy has to transit, but they are far less capable in the open sea (too much sea, too few aircraft, limited operational window) and are much more vulnerable to the defensive armament that can reasonably be added to a merchant vessel without compromising its primary function. Among aircraft weapons the best option, by far, is the air dropped torpedo. Unlike warships, which have the speed and maneuverability to often avoid the deployed weapon, merchant ships (in WW II generally possessed with top speeds of 10-12 knots) are unable to dodge a torpedo. One torpedo was sufficient to sink or cripple almost any merchant vessel used by any nation throughout the war.