I have been using Airtel, Vodafone,Idea & MTNL in Delhi for Voice calls for last 2 years now, AIrtel is decent but it has got black spots at quite a few places and if I may say the same goes for Vodafone & Idea. They have there pros & cons, in some places one is the Best Network and in the other you won't even find a single bar of network, I would suggest a demo at your home/office etc before getting a new connection, although I must say Idea has been the most consistent in network coverage since they bagged 900 Mhz Spectrum, vodafone has been the worst. As for MTNL just one word AVOID, in-doors you won't be able to make a single call, I can guarantee you that unless you live in high profile areas like where all the MPs live.
Haven't tried other networks but what I have heard from my colleagues is that Tata is pretty decent and so is MTS, haven't heard much about RCOM CDMA or GSM.
Aircel is also suppose to be decent, but I doubt it as I have personally faced major issues with Airtel and Vodafone and both are the oldest operators in Delhi and both have the largest numbers of towers besides being on 900 Mhz