Having seen the response (or lack thereof) to the "Does anyone play UT2K4?" thread I was a bit dubious as to wether or not anyone will respond to this, but since
I did a UT99 Version of this thread I thought it was only fair to do a UT2003/4 version.
Personally my favourite is the Ballistic Weapons mod for UT2004, link and mini-review
here, since like I noted in the review, the variety of improvements is truly astounding. Pity there aren't Ballistic Assault or Ballistic Invasion game modes, but I'm still happy with the ones they have provided.
What are your favourites?