Location?
Cheap call rates>MTNL/BSNL (Airtel most expensive)
Good reception>Airtel (MTNL/BSNL worst)
Prepaid > all operators provide prepaid services
Best Offers for sms charges > Vodafone (Never got any local sms package in Airtel)
If most of your friends/relative are on airtel, go for it as you may get some free airtel to airtel calls per month on some prepaid plans, otherwise Vodafone. On vodafone, you get packages for that much free sms if you recharge with a given amount, but on airtel , even after recharging with 31/35Rs. coupen all you get is sms rate reduced to 20p per message.