The new Mumbai airport is incredibly good. I love the architecture and the lighting. The ambience of the airport is just so calming. A close competitor is the Doha airport. Once again it has amazing architecture.
Delhi is a notch higher then Mumbai though both of these are now world class airports. The main advantage of the Mumbai airport is smooth transition, whether you are going for domestic flights or international flights, so that's a definite advantage. A close competitor is the Singapore's changi airport though, which is equally good.
So far from my travels in Asia the airport I think was the best in terms of cleanliness, architecture and choice shops was Singapore's Changi Airport as well. Actually the whole country is so clean and beautifully planned out. It can be hot sometimes and boring if you don't know where you're going but overall Singapore is a great place for travellers (and airport enthusiasts )
