Five attack submarines stand out as the best in the world today: Russia's Sierra II Class (Project 945A Kondor) and Yasen-Class (Project 885), the UK's Astute-class, and the U.S.'s Virginia-class and Seawolf-class.
The Russian Navy's Sierra II-class submarines, also known as Project 945A Kondor class, remain among the most expensive and deep-diving boats ever built. As a follow-up to the Sierra I, the subs were specifically developed for search-and-destroy missions against U.S. nuclear-powered ballistic missile submarines.
The Soviet Union didn't build additional Sierra II-Class or what is also called Project 945A Kondor Class submarines due to the high cost of the materials, and the difficulties in working with titanium.