Another vote for Airtel, followed by Vodafone. MTNL is absolute crap. Its network is barely in the reach. Have heard people saying that MTNL is by far the most affordable but my reasoning says that it is affordable only because it never serves the purpose of calling.
I used to have MTNL before (Dolphin postpaid). It was pretty good initially but off late, they had a lot of network issues.
So I switched to Vodafone postpaid. They are very good. Network is available everywhere & signal strength is excellent. Even their service is top notch. If at all any problem arises, they have solved it within 24 hours. So I am a happy Vodafone customer.