Das Boot and Hunt for Red October are great movies. But I grew up with the old back & whites stuff. My list of three favorites are:
Run Silent Run Deep, great picture. Too bad it was almost nothing like Beach's book though.
Operation Pacific, John Wayne. Need I say more?
The Enemy Below, classic destroyer vs submarine story.
Two other great submarine movies. That probably wouldn't otherwise be mentioned in this thread. Because they are not factual nor realistic are:
Operation Petticoat, the 1959 version with Cary Grant. Can you think of a better submarine comedy?
20,000 Leagues Under the Sea, the Disney 1954 version with James Mason as Captain Nemo. Who did love that one when they were a kid?
U-571 was a good movie if you didn't know anything about submarines.
I guess after movies will come television shows.
Voyage to the Bottom of the Sea.