What is your favorite QoL that's coming in 6.3?

[OM3GA-Z3RO]

I would say the Damage indicators but it looks really shit and it could clutter the screen with those icons, the Damage Info Plugin is far more superior than what they thought up.
Damage Info also has Casting indicators which the castbar is the color of the type of Attack it is and they have Positional indicators so if you hit your positional the Damage numbers color changes to Red.

Would've been much easier for the UI crew to just change the Damage numbers color

Yellow = Physical
Blue = Magical

Simple straightforward and no needless clutter. The Damage Indicators they are presenting is like a Chinese knockoff

Rant over and to answer the OP
Best UI change is the Favorite Bait and buff timers