I think that many people are answering in terms of where their child goes, or where they wish their child would go.
I wish my child had gone to Grace Episcopal Day School and that he went to St. Anselms now, I think those would be a great fit for him, but that doesn't mean I think it's "Big 3". To me "Big 3" means well big. Big school (lots of kids), big age range, big reputation (known widely), big curriculum (tries to do everything, sports, arts, academics etc . . . ), big applicant pool
I like the fact that GEDS is small, intimate, for a limited age group, has a neighborhood feel, accepts a wide range of kids, focuses on the things I think are most important, doesn't have an overwhelming feel to it. . . . That's what I wanted for him, and what made sense for our family. But other people want something different for their kid, they want the "big" experience. Nothing wrong with that.
So, I vote Sidwell, GDS, Cathedral Schools, with the addition of Potomac and Maret if you go to "Big 5".