TL;DR: Seoul, Korea. I loved living there.
I've not seen many cities on Earth. But, in making my choices, I'll share what Im considering.
Caveat: I've never been to any place on the African continent. This is an admitted blind spot. I've also been to only three European cities.
Not New York, Toronto, San Fransisco, Ottawa, Portland, Las Vegas, Washington D.C., Miami, Québec, Atlanta, or Halifax (sorry Nova Scotia, we love you!)
Not London (UK). Not Frankfurt. Not Sydney (Aus). Not Beijing. Not Kathmandu. Not KL, KK, Bangkok, or Singapore. Not Dubai. Not Colombo, Male, Kuwait, Doha, or Muscat (sorry Oman, we love you!)
No La Habana, ni Santiago de Chile, Cali, Arequipa, Cusco, y no á ciudad de Guatemala (disculpame mis hermanos de Guate!)
I think that the greatest cities on Earth have DEEP histories, offer diversity, and have been at the crossroads of the human condition.
Of the cities I've spent at least a week in (in no particular order):
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La ciudad de México, México. (Pyramids, gardens, art, and food)
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Istanbul, Türkiye (Hagia Sofia, Bosphorus, history)
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Seoul, Korea (functionality, history, food, mountains, music)
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Cartagena, Colombia (music, seaside, fortress, art)
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Beirut, Lebanon [at least, when (2012) and how (with well-connected locals) I was there. Certainly, it's faced repeated harms in the past 7 years.]
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Berlin, Germany (architecture, functionality, history, walking tour, art, festivals)
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Montréal, Québec (one Canadian city needed to be on)
Ones I've not been to, but might list if I had:
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Tehran, Iran (history)
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Barcelona, Spain (Al-Analous)
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Mumbai, India
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Hong Kong
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Tokyo, Japan