I can't speak to Boston and LA, but there are a couple of factors that would IMO give Philly an edge over DC, and also entry into any top 10 list.
1). Markets: Reading Terminal and the Italian Street Market, tough to find their equals in this country.
2). Several specialties that if not unique to Philly, they do very well there: Cheesesteak/soft pretzels/hoagies/PA Dutch specialties like scrapple.
3). A couple of restaurants that have one time or another been in the top 10 in the country: Susanna Foo, Le Bec Fin, Striped Bass.
4). Old standbys with very long histories (altho varying in quality over the years), such as Sansome Street Oyster House, Bookbinder's, 4th St. Deli, Standard Tap
Strong ethnic representatives, altho I'm personally familiar only with the Italian which seems to be fairly deep at a variety of price levels and styles.
DC, of course, has its attributes, and in terms of the diversity of its dining scene, I don't think it can be beat. (I've had my first Afghan, Ethiopian, Southern Indian, Malayan food there). My main gripe with DC, tho, is that many of the restaurants I've liked are gone by the time I return. Admittedly, it's a highly subjective opinion, particularly since I don't travel as much to DC as I used to, but my impression is that restaurants--good, bad, indifferent--have short shelf lives there.
"The fork with two prongs is in use in northern Europe. In England, they’re armed with a steel trident, a fork with three prongs. In France we have a fork with four prongs; it’s the height of civilization." Eugene Briffault (1846)