The two popular super mods are Real Fleet Boats(RFB) and Trigger Maru Overhauled(TMO).
Whichever you choose is user preference. I started with RFB, but ended up prefering TMO.
On top of those, if you want the best Campaign(in terms of Objectives given to you, and actualHistorical Accuracy in terms of Task Forces and Shipping in General) You want to get Run Silent Run Deep(RSRD). Everything else is up to you. Although most people recommend keeping the list of mods you have limited, so you don't end up with Mod related problems.
My current Mod setup is:
TMO2.0
TMO2.0 Beta Update
RSRD for TMO
RSRD Patch
SH4 Authentic Flag Mod
Convoy Routes TMO+RSRD
I also have Spax's German Sound mod for when I decide to play a German Career in UBM.
I also have TMO Nine Delta Two, which is a German Campaign set in the Indian Ocean/Black Sea. But I have yet to install it.
As a side note, expect to see less random merchant traffic in RSRD, compared to GWX. But you encounter a lot of Task Forces/ Invansion Forces that existed in real life.
