The National Mall
I was amazed that despite the Lincoln Memorial being kind of "standing room only" the southern end of it was largely empty, and I only had to wait for a few minutes for my opportunity.
Ten seconds earlier there was a woman in a colourful sari standing in the view - that was even better, really, but I never quite got the right moment, whereas the monuments are not moving and will happily sit still for minutes, or perhaps even weeks!