It has to be a stand-alone brand, so no asos or zalando type choices.
Mine is a close call between Cos and Uniqlo. I really like that monochromatic, vaguely Japanese, minimalist but striking look. I only rarely achieve it though.
It'll have to be Uniqlo, I don't really have a Cos lifestyle I don't think. I find the clothes aspirational though.
So what are you going for?