Black is the way to go. I stopped adding sugar 30 years ago and stopped adding milk 15 years ago. Personal taste of course.
In a lot of the places I have worked when deployed abroad, milk is not available, hence, I drink my coffee black.
However, whenever I am at home, or in western Europe, I will admit that I am partial to milk with my coffee.
As for sugar, (preferably brown sugar), I have only ever taken sugar (on occasion) with espresso.