I am using airtel since 2008 in Delhi NCR and it is best. The best thing I like is Airtel money, you can recharge without internet connection and smartphone. Network quality is also very good, but now a days, I think they have some network problem, mainly in east delhi. But still they are no. 1 and my first choice, their customer service are the best. You can manage your account on their website.They have a lot of customer friendly rechargepacks.
Finally, it is costly but effective.