UT2003, purely because I'm not the NWN type. You need to think about what you enjoy. All the games you listed are really very good but if you prefer RPGs over FPSs then obviously UT2003 is right out of the window, visa versa as well.
However, UT2003 is great fun, the engine is spectacular and feels a lot smoother than UT did (in my opinion). It's sped up mind you so if you prefered UTs slower pace over Q3s then it might not be to your taste...lots of long livety though, especially if you take it online.
Edit: As for mods, well UT2003 ships with a map editor and theres a lot of things you can do with the engine, a lot of mods are already in development and once again there are smaller mutators and so on that can be created. You won't get bored.