obviously GWX 2.1
included in the GWX 2.1 mod are several optional mods... i recommend these.
GWX main movie - 'Das Boot"
GWX no medals on crew
GWX Open hatch mod
GWX enhanced damage effects
other mods to get are
Life boats & Debris v3b
http://www.subsim.com/radioroom/showthread.php?t=134988
Interior diving angles - open hatch
http://www.subsim.com/radioroom/showthread.php?t=136442
OLCE2 (OLC Ubermod 1.6 Part 1 of 2)
OLC GUI Special (OLC Ubermod 1.6 part 2 of 2)
http://www.subsim.com/radioroom/showthread.php?t=134186
before you install any of this... try a couple of stock missions or just sail around a bit and get a feel for the stock game.
when you feel like you have the stock game pretty well pegged, load up those mods... it will blow you away