I will probably be keel hauled for posting this but my list is
1. K19 The actual experiences of the Hotel 1 was a bit less dramatic than the movie, but it still was a heroic crew in what they did,
2. Das Boot (perhaps this is second because I have seen it way too many times)
3. Enemy below
For entertainment purposes I like Destination Tokyo with Cary Grant. It is a true propaganda movie and not all that realistic but not all that bad either.
I am very happy to see that no one put "Crash Dive" on their list. More of a love triangle love story then a sub story <blach>
Torpedo Run with Glenn Ford is not bad. A pretty dramatic scene when he has to sink the Japanese transport carrying his wife and children.
I did not care for "Grey Lady Down" To me it was just another in the "Airport" disaster series with a sub.
I need to get a copy of "Torpedo Alley" and "We dive at dawn" for my sub library.