I haven’t traveled the world enough to give a definitive answer. I had a blast in Edinburgh, and Luxembourg City is a wonderful place to visit. Most large Canadian cities are delightful. I doubt any of the cities I have visited would be the ‘greatest’ on the planet, however.

I 100% doubt any city in the USA could qualify. They are far more alike than they are different.

As for New Yorkers frequently & loudly yelling their city is the greatest, I’m convinced it’s some kind of pathology similar to Stockholm Syndrome. This comedian explains it better than I can:

https://www.youtube.com/watch?v=nnxtvSMw49M