I have used airtel from many years and its quality and service is awesome .
Network - There is no excuse for signal strength . Many towers in countries there is low chance of dropping calls. clear conversation happen all the time .
CustomerServices - Fast and responsive customer care services and there is no issues happen.
Charges & Billing - In the charges, It has higher charges compare to others but give good quality services also. other services like caller tune, songs etc. have high rates
As per my view I give recommend for Airtel.