Sugar Intake

If you're not diabetic or with some other medical condition that leads you to need to limit sugar, I think the answer is: Low enough that you can still get adequate protein, fats, veggies and fruits within calorie goal, IMO.

I went over the MFP default sugar goal every single day while losing weight, but lost weight fine (and got good nutrition, because I think that's important; even though calories are what drive weight).

Personally, I find it easier to worry about getting the good nutrition I want into my diet within a sensible calorie level, rather than taking things out of it. When I do that, things fall into place. YMMV.

World Health Organization suggests getting less than 10% of daily calories from added sugar, but don't express much concern about the sugars inherent in things like fruit and no-sugar-added dairy foods (i.e., the sugars nature put in there).

Further, reading what they say about added sugar, the reasoning behind their recommendation is mainly (1) it's hard to get good nutrition on sensible calories with lots of added sugar in the picture; (2) it's easy to go over sensible calories while getting good nutrition, if there's lots of added sugar in there; and (3) people have more dental cavities when they eat too much sugar.

Some people find that sugar spikes their appetite, or is hard to moderate, which is a different question (and pretty individual). Of course people with relevant medical conditions should follow a registered dietitian's advice when it comes to sugar intake.