Question

Chennai is the greatest city in India change in to positive degree

Answer 1

Chennai is greater than most other the cities.

Answer 2

Given sentence (Chennai is the greatest city in India) is in Superlative degree.

To change into Positive degree, it can be rephrased as below, without changing its original meaning.

No other city in India is as great as Chennai.

Additional information:

Chennai is greater than any other city in India - Comparative degree

Hope it helps. Thanks!