[Sailor Steve]

First of all, whenever you ask for "must-haves" you risk getting 500 answers listing 500 different mods, every one considered a "must-have" by somebody.

Second, I assume you're using JSGME. If you are, and you get warnings of possible conflicts, look at what the conflicts are. A lot of the time it's just a readme, or something unimportant.

Third, you are much better off asking specific questions that general "what should I do?" types.

Okay, these are the mods I'm currently using:

Manually Installed Mods
GWX 3.0
GWX 3 Wil, St Naz and Schluese and other units
GWX Community Units
Das Boot Officers
Small Moon
Das Boot Sound Mod SH3
DBSM SH3 Speech Fix
Ultimate Aircraft Sounds
Spirit of Machines
Torpedo Fire (German)
No Rank or Medals on Crew
Remove GWX Compass
Simfeeling 6-Dials Mod
Clean Nav Map
Assisted Plotting Mod
English-to-German Names
Historically Accurate U-Boat Emblems
Halo Not Visible Through Body
U-505 Compass Graphics
Pascal's Port People
New Uniforms and Das Boot Clothes
Pascal's Realistic Blueprint Damage Screens
Remove Station Labels
SH3 Color Icons
Waterstream+Exhaust Kombimod
Modified Searchlight Beams
U-Boat Bases Map with New St. Nazaire
Lifeboats & Debris v4
Open Hatch v3.09
Open Hatch Conning Deck Cam Fix
SH4 Maptoos for SH3
Head 04 Fix
SH3 Officer Faces

JSMGE:
SH5 Water for GWX
TMTv2+ThomsenShips v4.4
GWX_DFa-Flag&Pens_2010
New_clearsky_clouds
FM30_UpDown
DD_OH_ConningDeckCam_Fix_10091
DD_OH_v3.09_Fix_for_Anvart_FM30_Updown
Conning Tower open Hatch093food
Torpedo damage Final
VIIB Early War Skin
Tuddleys Baracks Screen
WB's VIIA (SH3)
SRMO! - Seabed Repair Mod
SH4 Maptools for SH3-Stock
Sonar sounds
h.sie's Colored Hull Integrity Indicator
Torpedo_HAHD_1024_GWX
APD_Clemson
AFB_Fairy_Fulmar
Walrus by TP
Aces' Super Pens v4 GWX3 Version
VIIB_Splashwave_fix
Ricks_GWX_Rec_Man_Final
No continuous'Ship spotted' V1.2 for GWX3

I also have a bunch of mods in JSGME that I rotate out, such as Arctic/Atlantic/Med water, and plane and ship mods.

I also have my own set of Text Files that have everything exactly the way I want it. After installing any new mods I re-install Commands.cfg, 1024_Menu.ini, Dials.cfg.

I also use the following external tools:

SH3 Commander
SH3 Weather
SH3 Contacts

Note that I do not currently use FM's new interiors or IABL's MFM mod. My computer won't handle them. When I finally get a new one they will be installed.

Lastly, my list is short compared to some.