Greetings, and welcome to the World of Mods..
Firstly, NO ONE can suggest to you what mods are the best - it is purely subjective - some are excellent - some mediochre - each is a love-child of it's author who will probably (if you ask nicely) help you get it working!
Here is the BEST repository for SH3 files:
http://u-boot.realsimulation.com/ 
Some of the published mods will not work on your/my/anyones configuration - accept that as a fact of life (despite what you hear) and accept that there will be CTD's (crash to desk top!) when you install certain mods!
One poster suggests a certain 'Mod enabler' - this has Soooo screwed up my SH3 in the past that I refuse to do anything except manual mod insertions (go figure - it too is a MOD!)!
The best advice I can give you is in FIVE parts:
1. First Part - make a virgin copy of your ORIGINAL SH3 Data File.
2. Make a copy of the same having applied the UBISOFT 1.4b patch.
3. FULLY UNDERSTAND how each part of the Data file works before you MOD anything!
4. Add a Mod ONE AT A TIME and TEST!
5. Try a simple mods first - like adding 'gramophone' tunes or 'reskinning' your VIIc, or simply adding new missions!
- check the mission forums and downloads...
After you get confident with that maybe try crew uniforms - oh and and can recover from CTD's - away you go with the big stuff!
(remember also that some mods may actually restrict your game enjoyment as they have been created for real-sub-diehards!)
Remember - ABSOLUTELY NEVER does Bdu award merit badges for modifying Kriegsmarine property! :rotfl: :rotfl: :rotfl:
Happy Hunting...............