Chicago is a great place to take pictures on the street.  It's usually busy, but not stiflingly so, unless you're talking about the crowds around Wrigley Field during a Cubs home stand.  It's vibrant, a great place to walk (most places) and it has its own kind of brash beauty.  

My son lives in Chicago so I get up there often, and we do some photo walks occasionally.  I find these walks to be very relaxing, and they give me a unique perspective on this ever-changing city.