Dhaka is becoming one of the ____ cities in Asia.

One of the এর পরে সবসময় Superlative degree বসে। তাই busy-এর Superlative form busiest হবে।

Complete sentence: Dhaka is becoming one of the busiest cities in Asia.

আরও উদাহরণ:

• Razia was one of the happiest children in the orphanage.
• Feroz is one of the best boys in the class.