Lets do this backwards from Jim's being my first choice backwards in history although the sinking of the USS Thresher
was very important to future ship building programs called "Fail Safe"
I will go with these three important developments in submarine warfare
Number 1
1982, May 2 – British nuclear submarine HMS Conqueror sinks Argentine cruiser General Belgrano off the Falkland Islands.
It was the first sinking of any vessel by a nuclear-powered submarine in wartime and also the first time that a nuclear-powered submarine fired weapons in an act of war.
Number 2
The Frigate Bird test, which took place on May 6th, 1962
The test, code-named “Frigate Bird,” involved the launch of a Polaris missile from a submarine carrying a live nuclear warhead.
The missile traveled a thousand miles before streaking down on an uninhabited island in the middle of the Pacific Ocean, exploding with the force of 37 Hiroshima bombs.
Frigate Bird was the only US test of an operational ballistic missile with a live warhead. This test involved firing a Polaris A1 missile from a ballistic missile submarine.
The missile was launched by the USS Ethan Allen (SSBN-608) at 13:18 (local) from a position 1500 nm east-northeast of Christmas Island.
Number 3
The first Polaris boat, USS George Washington, fired a Polaris missile on 20 July 1960, and commenced its initial operational patrol on 16 November 1960