That all depends on what you like. What I have done is made a dual install. I have the American as one game and the German on the other. Download MultiSH4 and you can have as many installs as you like. You can find the link in the search section. That way you can pick and choose all you want.
I have Operation Monsoon on the German side and TMO on the US side. Environment mods are included in both mods, so I've not added any extra environment stuff. They sometimes cause problems with gameplay.
Other than Operation Monsoon, GWX (who you know from SH3),have a new mod on the way for SH4. On the US side TMO have a new beta out and Real Fleet Boat is also worth checking out- There's also Fall of the Rising Sun and Websters stuff.
The list is endless.
Good hunting
