I think Quake 3 is nearly the pinnacle of deathmatch/ Capture the flag thanks to perfectly balanced weapons and overall controls. UT was great, but mostly for the variety of game modes. I didn't think the weapons were properly balanced, but it did move well.
In UT2k3 I think they have the movement perfct, and bring the weapon's balance closer to quake so that skill begins to play more of a part in winning matches. Too many instant kills in UT really hurt the game as a serious shooter.
When Assault mode is added as a mod or by the Unreal team (they support thier games very well.) I think this will be the best FPS multiplayer shooter out there. (Unless CS2 or Quake 4 ante up a bit)