Best mod in an game (not moderator)

What is your favorite mod in any game I.E. Pixelmon for Minecraft or Calamity for Terraria.
Also fangames are allowed.

if you include those:
the base game(s) are LISA:The Painful and LISA:The Joyful.

the fangmes are LISA: The Hopeful, LISA:The Pointless (waiting for MU STILL), LISA: The Vegaful, LISA: The Pointless (Scholar of WIilbur Sin, a mod of pointless)