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To find out just how dangerous a question that can be, I suggest you read this old thread

Having read Steve's post and the thread he references: Hands down: it's the Baleo class submarine. Not only was it effective: 120 were built; 11 lost compared to the previous Gato class: 77 built; 20 lost and the Tench class: 29 built; 1 lost...but it executed the mission. Delivering victory is what a weapon platform is all about. During the Second World War, submarines comprised less than 2 percent of the U.S. Navy, but sank over 30 percent of Japan's navy, including eight aircraft carriers. More important, American submarines contributed to the virtual strangling of the Japanese economy by sinking almost five million tons of shipping—over 60 percent of the Japanese merchant marine. Victory at sea did not come cheaply. The Submarine Force lost 52 boats and 3,506 men. During the war the U-boats sank about 2,779 ships for a total of 14.1 million tons GRT. This figure is roughly 70% of all allied shipping losses in all theatres of the war and to all hostile action. The most successful year was 1942 when over 6 million tons of shipping were sunk in the Atlantic. However that only added up to approx. 5% overall of Allied merchant shipping-given the massive replacement ship-building programs.(Hitler's Uboat War vol I & II) the mythic 600,000+ tons-a-month sunk targeted to strangle Britain was never close to being consistently achieved. Germany lost 766 U-boats 1939-1945 out of 1154 that were commissioned in WWII. You simply cannot win a war with numbers like that... (My .02 cents)