Hi kabex, welcome aboard!
There are so many great mods available for so many aspects of the game that it really comes down to individual preference.
When you get GWX installed you will no doubt find that it already comes with a number of great mods for you to experiment with.
The best place to start for additional mods is probably here
http://www.subsim.com/radioroom/down...hp?do=cat&id=1
in Subsim's SH3 Downloads section. You may even find as you browse that some of the names ring a bell, and find mods that you remember using and liking before.
There's also a forum specifically for mods here
http://www.subsim.com/radioroom/forumdisplay.php?f=195
where new releases are announced as well as works in progress, plus many mods have existing threads that you can check out for info or to ask questions of the modders themselves or others who've used them before you.
If you are going to be using mods the best advice I can give is to download and install JSGME, a mod enabler that will let you install and uninstall most mods to your game folder without the need to go in and manually overwrite or add files to the various game directories involved. You'll find that program in the Downloads section.
Also a nice addition is SH3 Commander, which lets you manage a lot of game/crew/career options and launch the game from its own interface with your preferences already in place. It's also available from the Downloads section and has a dedicated thread in the Mods Workshop forum.
If you find you have questions about specific mods you'd like to use, just post them and I know folks will do their best to help you out with the info you need.
Happy Hunting!