NHC

chat heads, raised chat/hotbar, (toggleable)light levels and "info lines", hunger bar on a horse, fuel info

potion effect timers, heads hurt too, axolotl buckets, durability tooltips, open inventory while standing in nether portal

the armor statue companion UI, connected textures, better grass, unique dyes, no loading screen while reloading textures

map preview (great for map arts and bingo events), shulker box preview, name formatting station datapack, grass block

free-cam for easy schematic placements, improved statistics and advancements, potion effect timers

F3, recommended to keep V-Sync turned off - if you notice bad screen tearing(hopefully you won't), use Adaptive V-Sync.