Hard to choose. User Interface Organizer, FNV Mod Limit Fix, lStewieAl's Tweaks and Engine Fixes, all the script extender fixes, The Mod Configuration Menu, unofficial patches, but even if this didn't exist, if I could only choose 1 mod, I would definitely choose Willow... or Coito Ergo Sum.
I'm currently playing FNV with the new mod list and it's perfect. The quality of mods has changed tremendously over the years. Some are literally high tech.
I also want to mention one mod that you may not know and that will change your entire game.
https://www.youtube.com/watch?v=5uD7xuotSL0