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.
Russia's Sierra I-Class Submarine has a very unique titanium hull. That is an exotic feature the U.S. Navy can't match at the present, although for good reasons.