What US city is the most different from the rest?

None. And to be clear, I grew up in Philadelphia and I have a positive things to say about living and visiting there. New York has been the biggest city in the United States almost since the beginning, and that gives it about 200 years of being the biggest. Sometimes by a large margin. Things just accumulate when that happens. It has always been a very wealthy city. Even at times when the city also contain tremendous amounts of poverty, or when the city government was on the verge of bankruptcy, New York itself was valuable. Its buildings and real estate was valuable. New York was a major port, a major population center, and a major center of manufacturing, while also being the national financial center. The only thing it lacked that comparable Europeans lacked was the seat of national government. Until the motion picture industry diversified itself and then TV followed that path, it was arguably the entertainment center of the United States. Broadway, Tin Pan Alley, Harlem, early films, early TV, Studio 54. Other places can lay claim to the birth of a specific genre or two, but New York has put its stamp on centuries of American popular entertainment. Other cities have immigrant and ethnic enclaves with long influence and deep roots. New York has DOZENS, scores if you include the “former” ones that exist partly in memory these days. Other cities have a hinterland that includes smaller cities and towns, suburbs, a local rail network sometimes, things that aren’t in the city proper, but still get grouped into it. Santa Monica, Cambridge MA, “the tri-state area”. NYC ate five counties, and the commuter belt extends to five states (smaller than the west coast admittedly). These counties, now Burroughs, aren’t so much a neighborhood as each of them is a miniature collection of neighborhoods. Hell it’s not even miniature. Brooklyn was in the top 10 USA cities every year until New York absorbed it. The only thing that really comes close in terms of number of truly distinctive communities might be LA County taken as a whole. All of this wealth has seen the endowment of tremendous cultural institutions. Most of these were done by individual wealth or the collective wealth of the New York aristocracy, and not the government. It’s just crazy. Philly has a lot of good things going for it, but one of the bonus things that has going for it is a 90 minute train to New York City.