The popular brand within a certain area is determined by the pins you use, the threads you wear, and also the brands that are sold within the area. Usually, the brand sold within a district is the most popular, but it can change randomly at times.
Take outside the 104 Building for example. D+B is sold there so it is typically the most popular brand. But the more noise you defeat in that area allow what every pins you use to gain popularity as well.
Using pins of a certain brand definitely give you an advantage at times. If your pins are the most popular in an area when you enter a battle, they get a boost in strength (2x).
Other easy ways to tell what is the Popular Brand in that specific area are the signs and video screens. They always display what the top brand is at that moment. It is the easiest to tell what the popular brand is in the Scramble Crossing.
Also see » Weak Brand.