For me, the 21 really shines as a landscape lens. When you want or need a moderate wide fov (think 28 or 31 FF), this is the one I like. Great colours and contrast. (although there are those that claim these kinds of photos to be just hyperfocal shots you can just as well get with a P&S
).
Of course the landscapes you get to shoot depend on your surroundings. The one I posted just before was from a vacation in Austria (Alps). My 'natural' habitat is less spectacular: I live in what was called the "Low Countries". So, I get fields and slow rivers but also some nice old cities like Bruges, Ghent, Antwerp, Brussels, etc.
In the cities, I believe the barrel distortion can be somewhat of a hindrance. I would pick the 16-50 there, also a fine lens - provided it works and you get a good copy.
As for portraits, this really isn't my cup of tea: I'll leave this gladly to Ben, Diego and any others that are into this.