Airtel is the best network used all over the world and also it is very old network which has so many towers and whose network coverage is awesome.
I am using Airtel from abou 5 years and also it has no complaint at all. I used it for internet and calling both.
Airtel always gives the correct reason when the phone does not connects whereas other phones gives reason that the people you are trying to connect is out of network coverage.
There are so many choices of data packs and with many validities that gives more choices.
Also the customer support is awesome which always solves the issue. And also they solve the issue in even 24 hours.
There are many SMS packs at very cheap price.
I experienced Airtel and it was amazing. If you are thinking to change your mobile operator then go towards Airtel, it will not give any chance of complaint.