The two largest submarine fleets on the planet belong to the United States and the European Union. Lets look at the two of them.
The United States Navy has the largest force of nuclear submarines in the World. The Atlantic Fleet has a total of nine Ohio-class SSBNs, one Virginia-class SSN, two Seawolf-class SSNs, and 23 Los Angeles-class attack submarines. This force is probably the most powerful in the world on its own, without considering the submarines in the Pacific Fleet. SSNs have huge advantages in endurance over diesel-electric and air-independent propulsion submarines.
The European Union has a much more versatile force. The various countries involved have a large number of modern submarines of all types. Not only is there a potent force of SSNs and SSBNs, but the EU also carries a large force of modern non-nuclear submarines as well.
The United Kingdom arguably has the best submarine force on a boat-for-boat basis. This is a highly modern force with well-trained sailors. The United Kingdom has a force of seven Trafalgar-class submarines, five Swiftsure-class submarines, and four Vanguard-class SSBNs. This force will be augmented in the future with the Astute-class submarines, an improved version of the Trafalgar-class.
France follows with a smaller force of nuclear submarines. The four Le Triomphant-class SSBNs are coming in service, replacing the six LInflexible and the LRedoubtable classes. The French are also developing the Barracuda-class SSNs to replace the older Rubis-class SSNs (upgraded to the standard of the Amethyst, the fifth boat of that class).
Germany, which terrorized the Atlantic in 1914-1918 and in 1939-1945, has a potent force of diesel-electric submarines in the 12 Type 206-class submarines. These vessels are primarily designed to operate in the Baltic. But Germanys proposed eight Type 212-class submarines are a quantum leap in capability. These air-independent submarines are lethal vessels, which have more endurance than regular diesel-electric submarines, while also not having to deal with the cost and expense of a nuclear power plant.
Italy is also acquiring the Type 212 (two submarines), and also boasts the eight submarines of the Sauro-class. The latter vessels are optimized to operate in the Mediterranean. Greece has purchased three Type 214 submarines and have an option for a fourth. The Type 214 is a version of the Type 212 that will also use fuel cells. Greece also has a force of eight Type 209s, a widely-exported German design.
Spain is designing a larger version of the Scorpene, the S-80. The first batch of four submarines will replace the old Delfin-class submarines (slightly improved versions of the French Daphne-class). Spain is considering a second batch of four to replace the Agosta-class submarines as well. The Norwegians have six Ula-class coastal submarines, a design that has some German assistance. Denmark has three older Kobben-class submarines, and one Nacken-class submarine all of which are second-hand. Portugal rounds out the EU submarine force with two Daphne-class boats.
Which is better? That depends. In coastal waters, the sub forces of Germany, Norway, and Denmark will have a significant edge over the American SSNs. However, coastal submarines cannot operate very far from their coastal waters, and this takes 22 of the EUs submarines out of play when it comes to long distance operations. This means that the EU is bringing 18 SSNs, 8 SSBNs, and 24 SSKs versus the American force of 26 SSNs and 7 SSBNs. The EU has a slight quantitative edge, but the Royal Navy is the only one that can really match the quality of the U.S. Navys subs. It should be noted that Sweden has recently allowed the U.S. train against its new Gotland-class submarines with air-independent propulsion. The United States has routinely trained against Australian Collins-class submarines. The Australians have sometimes beaten the United States in exercises. The EU might have a slight edge, due to the large force of ocean-going SSKs, but it is an open question as to whether or not this makes it a superior force to the United States Navy all nuclear sub fleet. Harold C. Hutchison (email@example.com)
Submarines have been a major element in naval warfare since the First World War, where the German submarine U-9 sank three British cruisers in one day. The Battle of the Atlantic in the Second World War was fought from the first day of the war, to the moment Germany surrendered. One cannot control just the sea, one must control underneath the sea as well.