Hello,
I'm afraid there's no definitive answer to that question. It all boils down to
personal preferences. If you like fleetboats and the Pacific, then SH4 is the
one to get. If you like Uboats and Atlantic, then it's either SH3 or SH5.
Personally, I would say Silent Hunter 5. Why? Well, as polished as SH3 is,
thanks to modders, it lacks the immersion I get from SH5 and immersion is
for me #1 when it comes to sub games.
I'd say, go read some posts in SH3, 4 and 5 forums and see which suits you
the best.
EDIT: So.Much.Ninja'ing.