Currently, the United States, Great Britain, and Russia are on stealth parity (for the most part) in nuclear submarine technology, as those countries have the most experience.
Having said that, there is no quieter submarine than a diesel-electric boat running on the battery, and many nations use them. You literally have to be within rock throwing distance to hear them, unless you're using active sonar, which makes you an instant target. While they are easy targets when running their diesels, they are nightmares to contend with when they are on battery power.
While DE submarines have the range and stealth capability, battery power reduces tactical advantages in speed an maneuvering. As such, the stealth advantage is limited in nature, as battery power doesn't last forever.
USA has the most nuclear submarines
stealthier, stealthiest
German submarines were referred to as U-boats during the world wars.
It was Germany.
Britain, France, Germany, Italy and USA all used submarines during WW1
In almost every state in the country, in submarines, etc.
The worlds stealthiest airplane would probably be the size of your moms pubes
Germany
Take it from someone who has seen the American, British and Russian submarines in action -- the American's have the quietest submarines. The US says they do. The Russians (former USSR) say they do. The British say they do. Bottom line: It all depends upon who you ask.
During WWI, Germany had the biggest fleet of submarines and it was known and feared for its sneaky torpedo attacks on enemy ships.
It depends on the real question. If meant literally, then yes, women are allowed on submarines. They can visit some submarines. If the question is, "Are women allowed to serve/work on submarines?" then it depends on the country. In the Royal Navy (UK) they are not allowed to serve at present. The United States, however, is gearing up to allow some women officers to serve aboard U.S. Navy nuclear submarines.
USSR; US; UK; Germany & Japan.