In San Diego, the Britax Agile-B stroller is the most ubiquitous model. In DC, it is the Baby Jogger City Mini. In New York, the Uppababy. We'll have to see about Boston.
Stroller popularity is something I pay attention to these days. We are on a two-week trip up the East Coast with the baby, which offers ample comparative perspective. It also offers a very ill-timed context for her four-month sleep regression.