Other answers have pointed out that New York City is one of the largest cities in the world (although no longer the largest), and that the Doctor simply says this is the largest food source the Angels have had - not that it is the largest they could have.
I want to address the statement in the question that
there are are cities way better than New York that never sleep: Bangkok, Tokyo, Las Vegas.
Whether those other cities are better is a subjective question. However, the phrase "The city that never sleeps" is most closely associated with New York City through the Frank Sinatra song New York, New York
I want to wake up in a city that never sleeps