Most photographers nowadays will tell you that the most popular walk around lens is a 50mm or 35mm lens. The 50mm being a great focal length as it basically gives you the same field of view as the human eye. The 35mm is also great with being able to get a wider field of view to incorporate more of the background in your shots. Whether shooting digital or film, these are the standard prime lenses you'll see the most out on the street.
But as someone who doesn't always follow the standard line of thinking, my favorite walk around lens is the 85mm. Why the 85mm length? It forces me to focus on the details; to look closer for the subject of the shot. It can be harder to get what I want but it also forces me to think outside the box. Once I really started getting into photography, I realized that a lot of my photos are just wide landscapes. It's easy, and easy can become boring. Finding beauty in the details is harder and more rewarding because those details can be the things that people overlook.
Another reason the 85mm is good for me is, because of my anxiety, it keeps me at length while not feeling too much like a voyeur. On my K1000 the lens really isn't that much bigger than my 50mm. When walking around Philly recently I was able to shoot some candid shots of people while remaining out of the way. The ability to get shallow depth of field on someone crossing the street and not having to try and crop an image afterwards is great. Sometimes purposely making things more difficult can make things more rewarding.