I think Airtel & Hutch share the title of best mobile service providers.
AIRTEL:
+cheap gprs..anywhere in INDIA
+most no. of consumers
+Good plans
-costomer care ((wats that??

))
-network problems
Hutch:
+Good network( atleast here in Bombay).
+costomer care
+ADS
-GPRS is costly.
-tarrifs too..