For the biggest variety of chain and department stores, plus a few select designer boutiques, locals and tourists alike flock to Union Square [1] (btwn. Geary St., Stockton St., Post St., and Powell St.).
The shopping area includes more than just the square proper. More designer and brand-name stores cluster for several blocks in all directions.
Links:
[1] http://www.moon.com/destinations/california/san-francisco-and-the-bay-area/sights/union-square-and-nob-hill