Stop any Tokyoite on the street and odds are they're wearing at least one thing from Uniqlo, the ubiquitous retailer of indispensible basics. There are branches all over the city but this is the largest, stocking everything in the current line-up (which usually includes some collaborations with designers and artists).
If you're overwhelmed, start on the top floor, which has all the top-sellers in one place. The floor below carries limited-edition T-shirts.