Metro Berlin captured with the Canon 50mm 1.8
The Canon 50mm f/1.8 take fantastic images are fantastic sharp and very light to carry around when taking a long walk in the city. Compared to a zoom lens you will have to walk a bit more to get the right composition as you cannot zoom in or our on the lens. On the other hand you can set the aperture to 1.8 to minimise depth of field and really isolate your subject.
The lens is the cheapest in my camera bag and one of the sharpest and one of the most used!