I can think of many superlatives to describe New York City. However, "greatest" never would have crossed my mind.
I was going to say "biggest," but I didn't have time to check the facts. I briefly considered "most financialy influential," "most notorious," and maybe a couple others... I grabbed "greatest" half thinking that, in the generic sense, it really is.
I mean it's big. It's powerful. It's....great.
I could have said, "the city I love", but then again, that's not always true, either! But I do love it. Sort of like a person loves an abusive spouse. [g]