Change it positive form: London is one of the biggest cities of the world.

London is one of the biggest cities of the world- এই  sentence টি Superlative degree তে আছে। 
• Noun phrase 1 + be verb + one of the + Adjective (superlative) + Noun phrase 2 (plural) + phrase- এই ধরণের Superlative degree এর sentence কে Positive degree তে রূপান্তরের নিয়ম: 
- প্রথমে Very few বসবে + Noun phrase 2 (plural) বসবে + phrase + be + as + adjective + as + Noun phrase 1. 

তাই উত্তরটি হবে: Very few cities of the world are as big as London. 

Source: A Passage to the English Language by S M Zakir Hussain.