Welcome to Subsim Jonatron5.
You really can't go wrong with Silent Hunter 3-5. A lot of what you want will depend on the CPU processor you have, RAM memory and video card in your computer as the various simulations all have different requirements.
Aktung recommended SH3 with NYGM but you can also use SH3 with GWX or living silent hunter or the war aces add on. It's all a matter of taste and preferences.
The add on mods are really a matter of taste as well. It depends greatly on the level of realism you want. SH4 can be upgraded as well. Rockin Robbins is our resident SH4 Guru and TheBerbster and HW3 are our SH5 guru's but there are others here with insight into these simulations.
Read through the sections corresponding to the game you like and take a look at what the mods do and try them out. If the mods are enabled through JSGME, any modifications are reversible as well.
Enjoy your sims and welcome aboard.