My suggestion would be Reliance. Though I am not from delhi but like me you can check the postpaid plans (Find the Best Postpaid Mobile Plans at Reliance Communications specially Simple99) its too good...I am also thinking of chucking my Vodafone here in Bangalore after more than 5 years of usage.
Secondly, if you can, take up BSNL without any doubts. You will love it. No hidden costs, uninterrupted roaming...though customer care suck!