If you said of, you can learn something today! After a superlative like biggest, tallest, most famous, and so on, we use in if we’re talking about the relationship of an object to a place.
So it’s the biggest building in the world, the best restaurant in the country, the first Internet cafe in Nepal, etc.). If you’re talking about the relationship of something with other items of the same sort, you use of, so it’s the most beautiful of the royal palaces I have seen, the best of the lot, the most interesting of his books on China, etc.
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