The Airtel has the widest network coverage in India.wherever you go airtel network never ditches you. I have witnessed it many times even in remote areas of Himachal pradesh Like kihar and bhandal wher eno other network works Airtel just works fine and even provide decent Internet speed in such remote area.
The biggest problem with airtel is its high and sometime hidden charges it is common issue between airtel users that airtel so often activate some useless service on your phone without your concern and will start charging you for it monthly even after your continuous requests to them. Airtel is only advisable if you often visit secluded and remote areas because of its high cost bills and recharges.