I'm surprised nobody's mentioned
Exelon/First National Plaza. Great public space, with plenty of little nooks for a more intimate feeling and a TON of seating on the grand staircases to lure a lunchtime crowd. The designers resisted the urge to provide a blank slate like Daley or Federal Plaza, and surrounded the space with restaurants, a cafeteria, great art, and connections to the subways. It satisfies most of Jane Jacobs' and William H Whyte's criteria for successful urban spaces.
We can't have the vibrant year-round plaza culture of Italy or Spain because we don't have the climate, but that doesn't mean we can't create great spaces that fill up in the summer and are programmed in the winter.