Well it depends on a lot of things...
First of all, what PC you have and what game it can run well.
Second, what theater does appeal you more (Atlantic or Pacific)
If you have a modern, good computer, then
A) Pick SH3 for Atlantic theater, or SH4 with UBM expansion
B) Pick SH4 with UBM expansion for Pacific theater
If you have an old spec computer,
A) Pick Comand Aces of the Deep for Atlantic theater
B) Pick Silent Hunter 1 Commander's Edition for Pacific theater
You can also run in a medium spec PC Silent Hunter II for atlantic and Silent Hunter II+ Pacific Aces Expansion for pacific.
All of them are available at the subsim-com store
EDIT: Oh, and wellcome to subsim.com BTW