I have been using Airtel mobile sim for last 4 years.
It is very good simply because network is always available.
Wherever you go network is available. Airtel has the best 2g network among all the operators. Airtel plans are very costly but the network is very good. I used Airtel 2g internet.
The speed of Airtel 2g internet is very good in my area. Airtel is the first operator in India to introduce 4g services. Airtel is my best brand in mobile network. Most of my friends are using Airtel sim.