The music. Excluding night themes of outside areas which are pretty creatively bankrupt and Endwalker's hub music.

It used to be dungeons and job design. But job design stopped getting me engaged after Stormblood. And dungeons became dull chores when they stopped giving them distinct features.

Used to like the lore too, but Endwalker crapped all over it with its horrendous explanations to loose ends.

I guess my favorite bits of the game are ARR areas and MSQ pre 5.3.