Well, overall: Trigger Maru Overhauled 2.5 - for various reasons.
You get a manual, where all intends and purposes of the changes are written down. It is really packed with information about the game, and some background as well...
Just download it and read it:
http://www.subsim.com/radioroom/down...o=file&id=2999
Most important facts about it:
the game feels more realistic
the AI is more challenging
sounds and graphics are improved
reworked campaign
very user-friendly
There are mods that can do this better or that better, but overall, I think this mod is the greatest improvement you can have for your silent hunter 4 right now. It really made this game great for me. Always a MUST TRY!
http://www.subsim.com/radioroom/showthread.php?t=132330
There are other megamods that I like, but this is by far my favourite.