Hello! I just met a phrase "one of the most beautiful city" and was surprised, because I always thought that the right option is "one of the most beautiful cities" :) Is the second phrase also correct? And if both are, which one would you prefer to say and hear? :) Thank you!

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It's not correct. It's probably a typo or something.

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You are correct. It is some kind of mistake, if that is the whole context.

There is one possibility I can think of. In English it is common to use nouns as adjectives, and "city" can be used in this way. We can take about "city plans" or "city parks," for example. The following sentences are correct, because "city" is being used as an adjective, and "one of the most..." is not referring to "city," but to "plan" or "park" or "center:"

Washington D.C. has one of the most beautiful city plans in the United States.

Designed by Frederick Law Olmsted, New York's Central Park is one of the most beautiful city parks I know.

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You are correct. It should be one of the most beautiful cities. This is a very common mistake.