Shibuya Parco has been a trendsetter for decades and now, after a renovation finished in 2019, it is back in top form. There are global brands, but also Japanese ones, both internationally known (Comme des Gar莽ons and Yohji Yamamoto, etc) and less so (like Toga and Anrealage). You could pound the pavement in search of boutiques, or just come here.
It's great for food too: the basement food court has Delifucious and a branch of trendy ramen shop Mensho Tokyo.