Personally, I've always been partial to parking my ass on the sidewalk on 16th Street, about 20' up the hill from Castro. People drive by- but you don't actually see humans out of their cars. You can see, on a clear day, all the way to the East Bay (and about 45 minutes before genuine sunset, there's a building up in the hills that has a window that reflects light right at you from All That Way Over There, straight up 16th Street. Very cool!).
It's a good location for a bit of a stop and think. There's another location nearby that has much the same view, but is a bit too isolated: there aren't people around to be separate from, which is important. Or maybe I like that location even better and don't want to share it.
There are other locations that work for this, in any city. There are locations like this in other cities, too. My ABSOLUTELY favorite bit of being completely alone in an urban world, bar none, was walking from the East End to Trafalgar, to catch the night bus to West Ken at about 3 AM or so (I did this pretty regularly when I was living in London).
What is your favorite place to be alone among many people?