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've been using BSNL for a while now. No issues. Billing is good. Also using Vodafone as a secondary number. Before that I was using AT&T (later became Idea).
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.
I have used 2 providers till now Airtel and Idea. Airtel suits me with their cheaper corporate plans. Service wise, IMO, they are the best if you consider national range.
I am using Idea since 2008 and its prepaid.
So am blessed with no crosscheck of bills and hassle free service .Opted for DND so my cell phone hardly rings.