Ahoy! Welcome to Subsim!
Amazing.... depends what you want. I am using Fall of the Rising Sun. The eye candy is very nice, lots of new ships to shoot at, too many features for me to list since I am not on the team and they don't pay me
there is a sticky for it and the other main "Supermods" at the top of this forum. Read about them all and see what suits your fancy. Mods for the German side (Atlantic ops, etc.) are in the ATO section of the forums.
And the best tip any of us can give to someone starting with mods,
get and use JSGME. It is a program for enabling and disabling mods without messing up your original install. I re-installed the game more times then I care to admit because I messed something up and didn't back things up before I finally started using it.