Alot of it is personal preference. 50mm allows you to isolate your subject a little more, but you don't have a wide-berth on the scale focus. A 35mm and 28mm allow you to show your subject in their environment and allow for a greater DOF when scale focusing. Anything Wider than a 28mm forces you to get really close which can make for some really interesting shots,. I like a 21mm in crowds where I'm close by default. So currently, I use a 21mm, 28mm, and 35mm, and a 50mm. I use them for different applications in the street.