I've got two versions of
Das Boot, one being the extremely short version (I bought that years ago when there were no other versions available). I don't recommend this, on its own it's okay, but in comparison to more complete versions, you can see where it is lacking, in fact some references in it make little sense, as there is a lot of character exposition trimmed out of it.
The other version I have is the 'Director's Cut', that's good, but it is one of those films where you need to set aside time to watch it, as it's not exactly very short. With all that in mind, I intend to buy the original serialised version of it, that's the first version I ever saw, when it was on TV, and I think that's probably the one to have, as you can view it in manageable chunks, and see it in the episodic manner which it was originally geared towards in terms of pacing.
Don't forget
Run silent, Run deep too, another classic.
Chock