I travel a lot between Mumbai and Nagpur. I have tried various airlines on the same route. Here is why I think Indigo is the best compared to the others.
Kingfisher/Deccan
After the flight landed everyone was crammed into one bus in which the Ac did not work. It felt more like a local train.
Go Airways
While leaving from Mumbai we were made to wait in the bus to board the plane for 20 mins as the previous passengers had not left the airplane. Then the plane stood still for 45 minutes on the runway before take off. When questioned the flight attendant was quite rude and said that he was not allowed to speak on the matter.
JetLite
Although the flight was on time and the service acceptable. There was very little leg room in the plane this made the journey uncomfortable.
Air India
Need I say more.
Indigo
Out of all the times that I have traveled by Indigo the flight has never been late. Surprisingly it was always reached earlier than the indicated time. The legroom was adequate. The food that could be bought on board was tasty and had some variety. There were always two AC buses in good working condition even when the plane was not packed to capacity.
Hope they continue the good work.