Mostly I use the default UI (with a lot of the new features like name plates and floating numbers turned off most of the time).
But I also use:
S'rendarr to show buffs and debuffs on screen,
Azurah to enable me to move and resize the UI and to colour my targets name by 'reaction' (friendly, neutral, enemy or player) and let me know how much Ultimate I have (otherwise I forget to use it).
Fyrakin's Mini Map because I have no sense of direction,
and Harven's subtitles because they're more customisable than the default.