Dear; The population of Karachi is greater than any other city of Pakistan
or
The population of Karachi is the greatest among cities of Pakistan in sentence number 1 there is no need of using "of" as "in" is correctly used. While in sentence number 2, you have used superlative form of degree instead of comparative which also doesn't make any sense as far as my view is concerned. hope this is the answer of your question. Thanks

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